dt-bindings: regulator: add regulator-fixed-clock binding

dt-bindings: regulator: add regulator-fixed-clock binding

This adds the documentation to the compatible regulator-fixed-clock.
This binding is a special binding of regulator-fixed and adds the
ability to add a clock to regulator-fixed, so the regulator can be
enabled and disabled with that clock. If the special compatible
regulator-fixed-clock is used it is mandatory to supply a clock.

Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20190910062103.39641-4-philippe.schenker@toradex.com
Signed-off-by: Mark Brown <broonie@kernel.org>