media: dt-bindings: document the tegra CEC bindings

* Tegra HDMI CEC hardware

The HDMI CEC module is present in Tegra SoCs and its purpose is to

handle communication between HDMI connected devices over the CEC bus.

Required properties:

  - compatible : value should be one of the following:

	"nvidia,tegra114-cec"

	"nvidia,tegra124-cec"

	"nvidia,tegra210-cec"

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

	  mapped region.

  - interrupts : HDMI CEC interrupt number to the CPU.

  - clocks : from common clock binding: handle to HDMI CEC clock.

  - clock-names : from common clock binding: must contain "cec",

		  corresponding to the entry in the clocks property.

  - hdmi-phandle : phandle to the HDMI controller, see also cec.txt.

Example:

cec@70015000 {

	compatible = "nvidia,tegra124-cec";

	reg = <0x0 0x70015000 0x0 0x00001000>;

	interrupts = <GIC_SPI 3 IRQ_TYPE_LEVEL_HIGH>;

	clocks = <&tegra_car TEGRA124_CLK_CEC>;

	clock-names = "cec";

};