mailbox: Build Broadcom FlexRM driver as loadable module for iProc SOCs

By default, we build Broadcom FlexRM driver as loadable module for
iProc SOCs so that kernel image is little smaller and we load FlexRM
driver only when required.
Signed-off-by: NAnup Patel <anup.patel@broadcom.com>
Reviewed-by: NScott Branden <scott.branden@broadcom.com>
Signed-off-by: NJassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/Kconfig

@@ -166,7 +166,7 @@ config BCM_FLEXRM_MBOX
 	depends on ARCH_BCM_IPROC || COMPILE_TEST
 	depends on HAS_DMA
 	select GENERIC_MSI_IRQ_DOMAIN
-	default ARCH_BCM_IPROC
+	default m if ARCH_BCM_IPROC
 	help
 	  Mailbox implementation of the Broadcom FlexRM ring manager,
 	  which provides access to various offload engines on Broadcom