dt-bindings: add bindings documentation for PDC driver.

Add the device tree binding documentation for the PDC hardware
in Broadcom iProc SoCs.
Signed-off-by: Rob Rice <rob.rice@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Anup Patel <anup.patel@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/brcm,iproc-pdc-mbox.txt

+The PDC driver manages data transfer to and from various offload engines
+on some Broadcom SoCs. An SoC may have multiple PDC hardware blocks. There is
+one device tree entry per block.
+
+Required properties:
+- compatible : Should be "brcm,iproc-pdc-mbox".
+- reg: Should contain PDC registers location and length.
+- interrupts: Should contain the IRQ line for the PDC.
+- #mbox-cells: 1
+- brcm,rx-status-len: Length of metadata preceding received frames, in bytes.
+
+Optional properties:
+- brcm,use-bcm-hdr:  present if a BCM header precedes each frame.
+
+Example:
+	pdc0: iproc-pdc0@0x612c0000 {
+		compatible = "brcm,iproc-pdc-mbox";
+		reg = <0 0x612c0000 0 0x445>;  /* PDC FS0 regs */
+		interrupts = <GIC_SPI 187 IRQ_TYPE_LEVEL_HIGH>;
+		#mbox-cells = <1>;   /* one cell per mailbox channel */
+		brcm,rx-status-len = <32>;
+		brcm,use-bcm-hdr;
+	};