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>