gpio: moxart: convert to use basic mmio gpio library

This patch converts MOXART GPIO driver to use basic_mmio_gpio
generic library.

Signed-off-by: Kamlakant Patel <kamlakant.patel@linaro.org>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Jonas Jensen <jonas.jensen@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>