mailbox: mediatek: Add Mediatek CMDQ driver

This patch is first version of Mediatek Command Queue(CMDQ) driver. The
CMDQ is used to help write registers with critical time limitation,
such as updating display configuration during the vblank. It controls
Global Command Engine (GCE) hardware to achieve this requirement.
Currently, CMDQ only supports display related hardwares, but we expect
it can be extended to other hardwares for future requirements.

Signed-off-by: Houlong Wei <houlong.wei@mediatek.com>
Signed-off-by: HS Liao <hs.liao@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>