power: regulator: Add a driver for AXP PMIC regulators

power: regulator: Add a driver for AXP PMIC regulators

This driver handles most voltage regulators found in X-Powers AXP PMICs.
It is based on, and intended to replace, the regulator driver in TF-A.

AXP PMIC regulators can be divided into 6 categories:
 - Switches without voltage control => fully supported.
 - Single linear range => fully supported.
 - Two linear ranges, "step" and "2 * step" => fully supported.
 - Two linear ranges, "step" and "5 * step" => only the first range is
   supported. No boards are known to use the second range.
 - Non-linear voltage values => fully supported.
 - LDOs shared with GPIO pins => not supported.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>