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>

drivers/mailbox/Makefile

@@ -7,3 +7,5 @@ obj-$(CONFIG_PL320_MBOX)	+= pl320-ipc.o
 obj-$(CONFIG_OMAP2PLUS_MBOX)	+= omap-mailbox.o
 
 obj-$(CONFIG_PCC)		+= pcc.o
+
+obj-$(CONFIG_ALTERA_MBOX)	+= mailbox-altera.o