gpio: Add RDA Micro GPIO controller support

Add support for GPIO controller from RDA Micro. This GPIO controller
is an in house IP, developed by RDA Micro (now Unisoc) for the use in
RDA88* series of SoCs. There are multiple GPIO ports present in all SoCs,
each capable of addressing 32 GPIOs. But only first 8 pins have the
interrupt capability.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20191021064413.19840-4-manivannan.sadhasivam@linaro.org
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>