mailbox: Add Broadcom PDC mailbox driver

The Broadcom PDC mailbox driver is a mailbox controller that
manages data transfers to and from one or more offload engines.

Signed-off-by: Rob Rice <rob.rice@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>