mailbox: bcm-pdc: Changes so mbox client can be removed / re-inserted

Ensure that DMA is disabled, and pointers reset, when changing
DMA base addresses in pdc_ring_init().  This allows a mailbox client
to be re-inserted after being removed.  Otherwise, the DMA doesn't
restart so the client hangs while being reinserted.

Signed-off-by: Steve Lin <steven.lin1@broadcom.com>
Signed-off-by: Rob Rice <rob.rice@broadcom.com>
Reviewed-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>