ARM: PXA: Z2: Use only power button as wake source

It's very easy to press some keypad key when Z2 is closed,
so to prevent unexpected wakeups, use only 'Power' key to wake
Z2 from sleep.

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>