mailbox: Add generic mechanism for testing Mailbox Controllers

This particular Client implementation uses shared memory in order
to pass messages between Mailbox users; however, it can be easily
hacked to support any type of Controller.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/Makefile

@@ -2,6 +2,8 @@
 
 obj-$(CONFIG_MAILBOX)		+= mailbox.o
 
+obj-$(CONFIG_MAILBOX_TEST)	+= mailbox-test.o
+
 obj-$(CONFIG_ARM_MHU)	+= arm_mhu.o
 
 obj-$(CONFIG_PL320_MBOX)	+= pl320-ipc.o