regulator: axp20x: Support AXP806 variant

The X-Powers AXP806 PMIC has a new set of buck and LDO regulators, and
also a switch. The buck regulators support teaming into multi-phase
groups, with A+B, A+B+C, D+E groupings.

Some registers controlling DCDC converter work settings are at different
offsets. Deal with them as well.

Add support for this new variant.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>