pinctrl: sunxi: Move Allwinner A20 pinctrl driver to a driver of its own

Move the pin description to a driver specific to be.

This is the final 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.

We can finally remove that header, and remove all the driver part of the
pinctrl-sunxi core.

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