mailbox: Add support for APM X-Gene platform mailbox driver

X-Gene mailbox controller provides 8 mailbox channels, with
each channel has a dedicated interrupt line.

Signed-off-by: Feng Kan <fkan@apm.com>
Signed-off-by: Duc Dang <dhdang@apm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
diff --git a/drivers/mailbox/Makefile b/drivers/mailbox/Makefile
index 92435ef..b602ef8 100644
--- a/drivers/mailbox/Makefile
+++ b/drivers/mailbox/Makefile
@@ -17,3 +17,5 @@
 obj-$(CONFIG_BCM2835_MBOX)	+= bcm2835-mailbox.o
 
 obj-$(CONFIG_STI_MBOX)		+= mailbox-sti.o
+
+obj-$(CONFIG_XGENE_SLIMPRO_MBOX) += mailbox-xgene-slimpro.o