regulator: support multiple dummy fixed regulators

Currently regulator_register_fixed() uses a constant name to register a
fixed dummy regulator. This is sufficient in principle, since there is no
reason to register multiple such regulators. The user can simply supply all
consumers in one array and use it to initialise such a regulator. However,
in some cases it can be convenient to register multiple such regulators.
This is also a prerequisite for the upcoming patch, that will add a voltage
parameter to this function. The original function is provided as a wrapper
macro.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>