regulator: Allow modular build of mc13xxx-core

Since the MFD core for this device and the regulator drivers for these
devices can be built modular we should also support modular build of
the shared code for the regulator drivers, otherwise we try to link
built in code against modular code with unfortunate results.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>