crypto: brcm - remove BCM_PDC_MBOX dependency in Kconfig

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: 9d12ba8 ("crypto: brcm - Add Broadcom SPU driver")
Signed-off-by: Raveendra Padasalagi <raveendra.padasalagi@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Cc: stable@vger.kernel.org
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/crypto/Kconfig

@@ -655,7 +655,7 @@ source "drivers/crypto/virtio/Kconfig"
 config CRYPTO_DEV_BCM_SPU
 	tristate "Broadcom symmetric crypto/hash acceleration support"
 	depends on ARCH_BCM_IPROC
-	depends on BCM_PDC_MBOX
+	depends on MAILBOX
 	default m
 	select CRYPTO_DES
 	select CRYPTO_MD5