regulator: dummy: convert to use the faux device interface

The dummy regulator driver does not need to create a platform device, it
only did so because it was simple to do.  Change it over to use the
faux bus instead as this is NOT a real platform device, and it makes
the code even smaller than before.

Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/2025021027-outclass-stress-59dd@gregkh


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>