ARM: dts: am437x-idk: add gpio-based power key

AM437x IDK board has a User Switch which we can
program to whatever we want. Because this board
doesn't have a PMIC which can give us power button
presses, let's use this user switch as a gpio-keys
power button.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>