ARM: dts: sunxi: reference: Move the muxing back to the common DTSI

Now that all the SoCs using the tablet reference design DTSI are using the
same pinctrl naming scheme, we can move back the pinctrl phandles to the
main DTSI.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>