regulator: fixed: Don't supply voltage change ops when no GPIO is given

Rather than replicating the core support for always on regulators use
a different set of ops with none of the enable related operations provided
when we don't have any ops. This ensures that we automatically pick up
any enhanced support for such regulators that the core has such as the
warnings about regulation constraints that can't be used.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>