mailbox: Add HAS_DMA Kconfig dependency to BCM_PDC_MBOX

Add HAS_DMA Kconfig dependency to BCM_PDC_MBOX to avoid link
error on some platforms.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Rob Rice <rrice@broadcom.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>