mfd: early init for MFD running regulators

For MFDs running regulator cores, we really want them to be brought up early
during boot.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Mike Rapoport <mike@compulab.co.il>