regulator: axp20x: Add AXP15060 support

The AXP15060 is a typical I2C-controlled PMIC, seen on multiple boards
with different default register value. Current driver is tested on
Starfive Visionfive 2.

The RTCLDO is fixed, and cannot even be turned on or off. On top of
that, its voltage is customisable (either 1.8V or 3.3V). We pretend it's
a fixed 1.8V regulator since other AXP driver also do like this. Also,
BSP code ignores this regulator and it's not used according to VF2
schematic.

Describe the AXP15060's voltage settings and switch registers, how the
voltages are encoded, and connect this to the MFD device via its
regulator ID.

Signed-off-by: Shengyu Qu <wiagn233@outlook.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Shengyu Qu <wiagn233@outlook.com>
Link: https://lore.kernel.org/r/20230524000012.15028-4-andre.przywara@arm.com
Signed-off-by: Mark Brown <broonie@kernel.org>