mfd: Convert MFD drivers to use module_platform_driver

Factors out some boilerplate code for drivers doing the default thing
for platform driver registration. Drivers using platform_driver_probe
or an initcall other than module_init can't be converted.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>