regulator: provide missing regulator_ stubs

Change-Id: I8a5329284008c03705273dfa49042fc0e07b4b3d
Signed-off-by: Mark Stadler <mastadler@nvidia.com>
Reviewed-on: http://git-master/r/87068
Reviewed-by: Simone Willett <swillett@nvidia.com>
Tested-by: Simone Willett <swillett@nvidia.com>