Load switches are modeled as regulators but they just provide
the voltage of their parent input supply. So the drivers for
these switches usually don't provide a .get_voltage function
handler but there is code in the kernel that assumes that all
regulators should be able to provide its current voltage rail.

So, if the output voltage for a regulator is not available and
it has a parent supply, then pass the voltage of its parent.
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Mark Brown <broonie@linaro.org>