ARM: sunxi: Add pinctrl driver for Allwinner SoCs

The Allwinner SoCs have an IP module that handle both the muxing and the
GPIOs.

This IP has 8 banks of 32 bits, with a number of pins actually useful
for each of these banks varying from one to another, and depending on
the SoC used on the board.

This driver only implements the pinctrl part, the gpio part will come
eventually.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>