staging: mt7621-gpio: dt-bindings: add documentation for mt7621-gpio

This commit add missing dt bindings documentation for mt7621-gpio
driver. There is some missing stuff here about interrupts with is
not also being used in the mt7621.dtsi file. So just include in
staging a incomplete version before moving this to kernel's dt-bindings
place.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>