The maximum supported voltage for ldo_io# is 3.3V, but on cold boot
the selector comes up at 0x1f, which maps to 3.8V. This was previously
corrected by Allwinner's U-boot, which set all regulators on the PMICs
to some pre-configured voltage. With recent progress in U-boot SPL
support, this is no longer the case. In any case we should handle
this quirk in the kernel driver as well.

This invalid setting causes _regulator_get_voltage() to fail with -EINVAL
which causes regulator registration to fail when constrains are used:

[    1.054181] vcc-pg: failed to get the current voltage(-22)
[    1.059670] axp20x-regulator axp20x-regulator.0: Failed to register ldo_io0
[    1.069749] axp20x-regulator: probe of axp20x-regulator.0 failed with error -22

This commits makes the axp20x regulator driver accept the 0x1f register
value, fixing this.

The datasheet does not guarantee reliable operation above 3.3V, so on
boards where this regulator is used the regulator-max-microvolt setting
must be 3.3V or less.

This is essentially the same as the commit f40d4896 ("regulator:
axp20x: Fix axp22x ldo_io registration error on cold boot") for AXP22x
PMICs.

Fixes: a51f9f46 ("regulator: axp20x: support AXP809 variant")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>