sunxi: gpio: Enable support for H6 pin controller

The Allwinner H6 pin controller is not really special, at least not when
it comes to normal GPIO operation.

Add the H6 compatible strings to the list of recognised strings, to make
GPIOs work for H6 boards.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com> # Pine-H64
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>