regulator: core: Allow to get voltage count and list from parent

Load switches are modeled as regulators but they just provide
the voltage of their parent input supply. So, the drivers for
these switches usually neither provide a .list_voltage handler
not set a .n_voltages count. But there is code in the kernel
that assumes that all regulators should be able to provide this
information (e.g: cpufreq and mmc subsystems).

If the voltage count and list are not available for a regulator
and it has a parent input supply, then use the parent values.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Mark Brown <broonie@linaro.org>