ARM: dts: Add power button support for igepv5

Add power button support for igepv5.

Cc: Agustí Fontquerni i Gorchs <afontquerni@iseebcn.com>
Cc: Enric Balletbo Serra <eballetbo@gmail.com>
Cc: Javier Martinez Canillas <javier@osg.samsung.com>
Cc: Pau Pajuel <ppajuel@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>