regulator: core: Pull non-DT supply mapping into regulator_dev_lookup()

Ensure we always apply the supply mapping when doing a lookup rather than
only doing it in non-DT cases, ensuring that regulators with supplies
specified in the regulator_desc can find their supplies on non-DT systems
and generally making the code more obvious.

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