ARM: dts: am335x-chiliboard: Support power button

On chiliBoard power button is connected to TPS65217. It signals power
button presses by asserting interrupt output and allows to read the
state by i2c bus.

Handle TPS65217 interrupts and enable notifications of power button
presses.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>