ARM: 6415/1: ux500: DB5500 mailbox driver

This is a driver for the mailboxes used to communicate with the
DB5500 modem portions.

Signed-off-by: Stefan Nilsson XK <stefan.xk.nilsson@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>