mailbox: Add Altera mailbox driver

The Altera mailbox allows for interprocessor communication. It supports
only one channel and work as either sender or receiver.

Signed-off-by: Ley Foon Tan <lftan@altera.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index aaa039d..f2f0faf 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -563,6 +563,12 @@
 L:	linux-alpha@vger.kernel.org
 F:	arch/alpha/
 
+ALTERA MAILBOX DRIVER
+M:	Ley Foon Tan <lftan@altera.com>
+L:	nios2-dev@lists.rocketboards.org (moderated for non-subscribers)
+S:	Maintained
+F:	drivers/mailbox/mailbox-altera.c
+
 ALTERA TRIPLE SPEED ETHERNET DRIVER
 M:	Vince Bridgers <vbridger@opensource.altera.com>
 L:	netdev@vger.kernel.org