Now that we have axp22x.dtsi describing common axp22x hardware, use
it and reference the nodes instead of declaring the whole tree.

Also drop the "always-on" from the vdd-gpu regulator, since we don't
support the GPU anyway.

And add a regulator reference for cpu0.
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>