dt-bindings: display: Add STM32 LTDC driver

* STMicroelectronics STM32 lcd-tft display controller

- ltdc: lcd-tft display controller host

  must be a sub-node of st-display-subsystem

  Required properties:

  - compatible: "st,stm32-ltdc"

  - reg: Physical base address of the IP registers and length of memory mapped region.

  - clocks: A list of phandle + clock-specifier pairs, one for each

    entry in 'clock-names'.

  - clock-names: A list of clock names. For ltdc it should contain:

      - "lcd" for the clock feeding the output pixel clock & IP clock.

  - resets: reset to be used by the device (defined by use of RCC macro).

  Required nodes:

    - Video port for RGB output.

Example:

/ {

	...

	soc {

	...

		ltdc: display-controller@40016800 {

			compatible = "st,stm32-ltdc";

			reg = <0x40016800 0x200>;

			interrupts = <88>, <89>;

			resets = <&rcc STM32F4_APB2_RESET(LTDC)>;

			clocks = <&rcc 1 CLK_LCD>;

			clock-names = "lcd";

			port {

				ltdc_out_rgb: endpoint {

				};

			};

		};

	};

};