mailbox: bcm-flexrm-mailbox: add depends on ARCH_BCM_IPROC

The Broadcom FlexRM Mailbox is only present in the Broadcom IPROC SoCs.
Add depends on ARCH_BCM_IPROC to BCM_FLEXRX_MBOX.

Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>