drivers: regulator: add support for AXP2101 power management IC

Add initial support for the AXP2101 power management IC from X-powers.
Remark: only DC/DC1 and ALDO have been tested on real hardware.

Co-authored-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>
Co-authored-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

Signed-off-by: Lothar Felten <lothar.felten@gmail.com>