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

It adds the dt-bindings document for ZTE ZX IRDEC remote control
block.
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NSean Young <sean@mess.org>
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/devicetree/bindings/media/zx-irdec.txt

+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>;
+	};