regulator: fixed: Mark regulator-fixed-domain as deprecated

regulator: fixed: Mark regulator-fixed-domain as deprecated

A power domain should not be modelled as a regulator, not even for the
simplest case as recent discussions have concluded around the existing
regulator-fixed-domain DT binding.

Fortunately, there is only one user of the binding that was recently added.
Therefore, let's mark the binding as deprecated to prevent it from being
further used.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20210705133441.11344-1-ulf.hansson@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>