QTI Snapdragon Display Engine (SDE) DP-MST sideband message emulation driver

Required properties:
- compatible:		"qcom,dp-mst-sim"

Optional properties:
- qcom,dpcd-max-rate:     A u32 property defines dpcd reg 0x1.
- qcom,dpcd-max-lane:     A u32 property defines dpcd reg 0x2.
- qcom,dpcd-mst:          A u32 property defines dpcd reg 0x21.
- qcom,dpcd-regs:         A <addr,val> array property defines dpcd reg addr
                          and val.
- qcom,voltage-swing:     A u32 property that overwrite dpcd reg 0x103~0x106.
- qcom,pre-emphasis:      A u32 property that overwrite dpcd reg 0x103~0x106.
- qcom,link-training-cnt: A u32 property defines physical link training cnt
                          before switching to simulation mode.
- qcom,skip-edid:         A boolean property defines if edid will read from
                          internal edid buffer.
- qcom,skip-dpcd-read:    A boolean property defines if dpcd will read from
                          internal dpcd buffer.
- qcom,skip-link-training: A boolean property defines if link training will
                          be skipped.
- qcom,skip-dpcd-write:   A boolean property defines if dpcd write will be
                          ignored.
- qcom,skip-hpd:          A boolean property defines if HPD will be ignored
                          and always set high.
- qcom,skip-mst:          A boolean property defines if MST sideband message
                          need to be simulated.

Each child node represents a port at root branch, with properties:
- qcom,edid:               A u8 array property defines EDID raw data. Once
                           defined, below properties will be ignored. For any
                           mode with horizontal width > 4095, only EDID raw
                           data format is supported.
- qcom,mode-h-active:      A u32 property defines the horizontal active size.
- qcom,mode-h-front-porch: A u32 property defines the horizontal front porch.
- qcom,mode-h-pulse-width: A u32 property defines the horizontal pulse.
- qcom,mode-h-back-porch:  A u32 property defines the horizontal back porch.
- qcom,mode-h-active-high: A boolean property if horizontal polarity is high.
- qcom,mode-v-active:      A u32 property defines the vertical active size.
- qcom,mode-v-front-porch: A u32 property defines the vertical front portch.
- qcom,mode-v-pulse-width: A u32 property defines the vertical pulse width.
- qcom,mode-v-back-porch:  A u32 property defines the vertical back porch.
- qcom,mode-v-active-high: A boolean property if vertical polarity is high.
- qcom,mode-refresh-rate:  A u32 property defines vertial refresh rate.
- qcom,mode-clock-in-khz:  A u32 property defines clock in kHz.

Example:

/ {
	...

	sde_dp_mst_sim: qcom,dp-mst-sim {
		compatible = "qcom,dp-mst-sim";

		port@0 {
			qcom,mode-h-active = <1920>;
			qcom,mode-h-front-porch = <88>;
			qcom,mode-h-pulse-width = <44>;
			qcom,mode-h-back-porch = <148>;
			qcom,mode-h-active-high;
			qcom,mode-v-active = <1080>;
			qcom,mode-v-front-porch = <4>;
			qcom,mode-v-pulse-width = <5>;
			qcom,mode-v-back-porch = <36>;
			qcom,mode-v-active-high;
			qcom,mode-refresh-rate = <60>;
			qcom,mode-clock-in-khz = <148500>;
		};

		port@1 {
			qcom,mode-h-active = <1920>;
			qcom,mode-h-front-porch = <88>;
			qcom,mode-h-pulse-width = <44>;
			qcom,mode-h-back-porch = <148>;
			qcom,mode-h-active-high;
			qcom,mode-v-active = <1080>;
			qcom,mode-v-front-porch = <4>;
			qcom,mode-v-pulse-width = <5>;
			qcom,mode-v-back-porch = <36>;
			qcom,mode-v-active-high;
			qcom,mode-refresh-rate = <60>;
			qcom,mode-clock-in-khz = <148500>;
		};
	};
};