media: dt-bindings: add bindings document for zx-irdec

IR Decoder (IRDEC) on ZTE ZX family SoCs

Required properties:

 - compatible: Should be "zte,zx296718-irdec".

 - reg: Physical base address and length of IRDEC registers.

 - interrupts: Interrupt number of IRDEC.

Exmaples:

	irdec: ir-decoder@111000 {

		compatible = "zte,zx296718-irdec";

		reg = <0x111000 0x1000>;

		interrupts = <GIC_SPI 111 IRQ_TYPE_LEVEL_HIGH>;

	};