regulator: axp20x: use defines for masks

The AXP20X driver currently has several masks defined throughout the
code. Use nice defines to make them clean and more descriptive.

Additionally include bitops.h, which was missing before, and sort
headers.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
Signed-off-by: Priit Laes <plaes@plaes.org>
Signed-off-by: Mark Brown <broonie@kernel.org>