Some of the Broadcom iProc SoCs have FlexRM ring manager
which provides a ring-based programming interface to various
offload engines (e.g. RAID, Crypto, etc).

This patch adds a common mailbox driver for Broadcom FlexRM
ring manager which can be shared by various offload engine
drivers (implemented as mailbox clients).
Reviewed-by: NRay Jui <ray.jui@broadcom.com>
Reviewed-by: NScott Branden <scott.branden@broadcom.com>
Reviewed-by: NPramod KUMAR <pramod.kumar@broadcom.com>
Signed-off-by: NAnup Patel <anup.patel@broadcom.com>
Signed-off-by: NJassi Brar <jaswinder.singh@linaro.org>