gpio: loongson: Add Loongson-3A/3B GPIO driver support

Improve Loongson-2's GPIO driver to support Loongson-3A/3B, and update
Loongson-3's default config file.

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>