staging: mt7621-dts: convert to gpio-keys

Now that gpio-interrupts work correctly, we
can use gpio-keys instead of gpio-keys-polled
for the single push-button on the gbpc-1.

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>