gpio: pxa: add PXA1928 gpio type support

Add support for PXA1928 GPIOs. The PXA1928 adds a 6th bank from previous
generations.

Signed-off-by: Jing Xiang <jxiang@marvell.com>
Signed-off-by: Xiangzhan Meng <mengxzh@marvell.com>
[robh: ported to 3.19 from vendor kernel]
Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Alexandre Courbot <gnurou@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>