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.  On some chips, the PDC functionality is
handled by the FA2 (Northstar Plus).

Required properties:
- compatible : Should be "brcm,iproc-pdc-mbox" or "brcm,iproc-fa2-mbox" for
  FA2/Northstar Plus.
- 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@612c0000 {
		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;
	};