pinctrl: sunxi: Move the pins definitions to a separate header

It will allow us to have a cleaner separation between the data needed by
the driver to work, and the core logic of the driver in itself, and will
allow having too much noise in the core driver in the future.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>