staging: mt7621-gpio: dt-bindings: complete documentation for the gpio

This commit reviews and complete documentation for gpio related stuff
in the mt7621 device. It should be complete now.

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