regulator: axp20x: Migrate to regulator core's simplified DT parsing code

A common simplified DT parsing code for regulators was introduced in
commit a0c7b164

 ("regulator: of: Provide simplified DT parsing
method"). This is very similar to our own code, so get rid of ours
and use the common code.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>