pinctrl: sunxi: Move Allwinner A31 special pins driver to a driver of its own

Move the pin description to a driver specific to be. This is one more step
toward retiring pinctrl-sunxi-pins.h that used to define all the pins for all
the Allwinner SoCs in a single header, that would have in turn result in having
these structures in the final binary as many times as the header was included.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>