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: Anup Patel <anup.patel@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>