ARM: 6353/1: nomadik-gpio: add SLPM register handing for DB8500v2

The SLPM register enables/disables wakeup detection on DB8500v2.

Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>