MIPS: Cleanup Loongson-2F's gpio driver

This cleanup is prepare to move the driver to drivers/gpio. Custom
definitions of gpio_get_value()/gpio_set_value() are dropped.

Acked-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>