pinctrl: sunxi: Add Allwinner A20 clock output pin functions

This patch adds the clock output pin functions on the A20.
The 2 pins can output a configurable clock to be used by
external modules. This is used on the CubieTruck, to supply
a 32768 Hz low power clock to the onboard Wifi+BT module.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>