regulator: fixed: add possibility to enable by clock

This commit adds the possibility to choose the compatible
"regulator-fixed-clock" in devicetree.

This is a special regulator-fixed that has to have a clock, from which
the regulator gets switched on and off.

Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Link: https://lore.kernel.org/r/20190910062103.39641-2-philippe.schenker@toradex.com


Signed-off-by: Mark Brown <broonie@kernel.org>