regulator: axp20x: Use linear voltage ranges for AXP20X LDO4

AXP20X datasheet lists the possible voltage settings for LDO4, so
it was implemented using a voltage table. Upon closer examination,
the valid voltages can be mapped into 3 linear ranges.

Move AXP20X LDO4 to use linear ranges. The supporting code can be
reused with later AXP8xx PMICs, which have a number of regulators
that have 2 linear ranges.

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