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: 9d12ba86f818 ("crypto: brcm - Add Broadcom SPU driver")
Signed-off-by: Raveendra Padasalagi <[e-mail]>
Reviewed-by: Ray Jui <[e-mail]>
Reviewed-by: Scott Branden <[e-mail]>
Cc: [e-mail]
Signed-off-by: Herbert Xu <[e-mail]>

drivers/crypto/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