staging: mt7621-gpio: use custom xlate function

The default gpio.of_xlate function assumes there is one gpio chip
for each devicetree node. Device tree had changed to only use one node,
which corresponds to 3 different gpio chips now. For that approach
to work we need a custom xlate function.

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