staging: mt7621-gpio: replace 'mtk' to use correct one 'mediatek'

Gpio driver is using mtk and there is already 'mediatek' binding
defined for this maker. Update driver to use it instead the custom
one 'mtk'.

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