ARM: dts: sun6i: add p2wi controller node to dtsi

The p2wi controller has only one possible pinmux setting. Use it by
default in the dtsi, instead of having to set it in each board's dts.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
[wens@csie.org: reformat commit title; rename p2wi pins and use as default]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>