power: regulator: add AXP313 support

The X-Powers AXP313a is a small PMIC with just three buck converters and
three LDOs, one of which is actually fixed (so not modelled here).

Add the compatible string and the respective regulator ranges to allow
drivers to adjust voltages.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>