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 case of regulator-fixed where a clock has to
be used to switch the regulator on and off.

Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>