[ARM] 4970/1: tosa: correct gpio used for wake up.

TOSA_GPIO_ON_KEY can't wakeup the device. But the board
provides TOSA_GPIO_POWERON which is OR of (on_ac) and (on_button).
Use it for wake up.

Signed-off-by: Dmitry Baryshkov <dbaryshkov@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>