regulator: consumer: Add missing stubs to regulator/consumer.h

Add missing stubs to regulator/consumer.h in order to fix COMPILE_TEST
of the kernel. In particular this should fix compile-testing of OPP core
because of a missing stub for regulator_sync_voltage().

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20210120205844.12658-1-digetx@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>