pinctrl: sunxi: add clock support

For the port controller to work, we need to enable the apb0_pio gate.
This commit adds the ability to enable one clock specified on the device
tree to the pinctrl driver.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>