staging: mt7621-dts: update gpios related entries to use 'mediatek'

Gpio driver for mt7621 is using 'mtk' as binding but in the kernel
is already defined one for this maker which is 'mediatek'. Update
device tree to use the correct one.

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