pinctrl: sunxi: support multiple pin controller

Add support for multiple pin controller instances.

First remove the static definition of the sunxi gpio chip struct and fill
the dynamically struct instead.
Then define a new pin_base field in the sunxi_pinctrl_desc which will be
used to specify the gpiochip base pin.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>