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>
diff --git a/drivers/mailbox/Makefile b/drivers/mailbox/Makefile
index 66c38e3..ace6fed 100644
--- a/drivers/mailbox/Makefile
+++ b/drivers/mailbox/Makefile
@@ -6,6 +6,8 @@
 
 obj-$(CONFIG_ARM_MHU)	+= arm_mhu.o
 
+obj-$(CONFIG_PLATFORM_MHU)	+= platform_mhu.o
+
 obj-$(CONFIG_PL320_MBOX)	+= pl320-ipc.o
 
 obj-$(CONFIG_OMAP2PLUS_MBOX)	+= omap-mailbox.o