pinctrl: sunxi: Move the reset handling functions out of the core

The way that reset is handled right now is that it is made optional for every
pinctrl driver, while actually, it isn't used at all for the main pin
controllers so far, and while it's mandatory for the A31's secondary pin
controller.

Move the reset functions out of the core and in the driver, where they can be
made mandatory.

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