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>

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>

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>