regulator: core: don't require a supply when supply_name is specified

Regulator drivers may specify regulator_desc->supply_name which
regulator_register() will use to find the supply node for a regulator.
If no supply was specified in the device tree or the supply has yet
to be registered regulator_register() will fail, deferring the probe
of the regulator.  In the case where no supply node was specified in the
device tree, there is no supply and it is pointless to try and find one
later, so go ahead and add the regulator without the supply.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>