pinctrl: ingenic: Merge GPIO functionality

Merge the code of the gpio-ingenic driver into the pinctrl-ingenic
driver.

The reason behind this, is that the same hardware block handles both pin
config / muxing and GPIO.

ingenic_gpio_probe() have been marked as __init, but for the most part,
the code is the exact same as what it was in the gpio-ingenic driver.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>