ARM: sunxi: dt: Add PIO controller to A31 DTSI

The A31 has a different set of pins than the one found on the A10 and
A13. Now that we have support for the A31 pin set in the pinctrl driver,
we can enable it in the DTSI with its own compatible.

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