gpio: sa1100: use sa11x0_gpio_set_wake()

Use sa11x0_gpio_set_wake() to set the PWER register, as provided by
Dmitry some time back.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>