plat-nomadik: change sleep/wakeup setting in GPIO SLPM register

This patch fixes a bug when setting SLPM register for DB8500.
When calling__nmk_gpio_set_slpm(...) offset to GPIO is now used
instead of the GPIO number itself.

Signed-off-by: Rikard Olsson <rikard.p.olsson@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>