mailbox: Add Platform Message-Handling-Unit variant driver

Add Message-Handling-Unit driver for platform variants as mailbox controller.
Actually, only the Amlogic Meson GXBB SoC MHU is supported.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>