regulator: axp20x: Use of_match name as default regulator name

Originally the helper macros used uppercase regulator names, which
are primarily used to expand to the regulator ID enum, as the default
names. This is aestheticly unpleasent.

Since the of_match bits are the same, just lowercase, use that as the
default names instead.

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