SPU driver is dependent on generic MAILBOX API's to
communicate with underlying DMA engine driver.

So this patch removes BCM_PDC_MBOX "depends on" for SPU driver
in Kconfig and adds MAILBOX as dependent module.

Fixes: 9d12ba86 ("crypto: brcm - Add Broadcom SPU driver")
Signed-off-by: NRaveendra Padasalagi <raveendra.padasalagi@broadcom.com>
Reviewed-by: NRay Jui <ray.jui@broadcom.com>
Reviewed-by: NScott Branden <scott.branden@broadcom.com>
Cc: stable@vger.kernel.org
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>