regulator: axp20x: Prepare support for multiple AXP chip families

Rework the AXP20X_ macros and probe function to support the several chip
families, so that each family can define it's own set of regulators.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
[wens@csie.org: Support different DC-DC work frequency ranges]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>