[media] Documentation: DT: add bindings for ST DELTA

* STMicroelectronics DELTA multi-format video decoder

Required properties:

- compatible: should be "st,st-delta".

- clocks: from common clock binding: handle hardware IP needed clocks, the

  number of clocks may depend on the SoC type.

  See ../clock/clock-bindings.txt for details.

- clock-names: names of the clocks listed in clocks property in the same order.

Example:

	delta0 {

		compatible = "st,st-delta";

		clock-names = "delta", "delta-st231", "delta-flash-promip";

		clocks = <&clk_s_c0_flexgen CLK_VID_DMU>,

			 <&clk_s_c0_flexgen CLK_ST231_DMU>,

			 <&clk_s_c0_flexgen CLK_FLASH_PROMIP>;

	};