pinctrl: sunxi: Fix gpio_set behaviour

The current gpio_set function is ignoring the previous value set in the
GPIO value register, which leads in erasing the values already set for
the other GPIOs in the same bank when setting the value of a given GPIO.

Add the usual read/mask/write pattern to fix this brown paper bag bug.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>