dt/bindings: gpio: add compatible string for marvell,pxa1928-gpio

Add a new compatible string for PXA1928 GPIO controller. The IP block is
same as prior chips with a 6th bank added.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>