regulator: axp20x: Remove voltage readout support for switch regulators

Switch-type regulators, such as DC1SW on AXP22X, are a secondary output
from DCDC1. They are just an on/off switch, and the driver should not
try to read its voltage directly from the DCDC1 control registers.

Instead, the core will pass down the voltage from the regulator supply
chain.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>