regulator: mc13xxx-regulator-core: Fix the build when driver is selected as module

Fix the following build error when mc138xxx driver is built as module:

ERROR: "mc13xxx_parse_regulators_dt" [drivers/regulator/mc13892-regulator.ko] undefined!
ERROR: "mc13xxx_get_num_regulators_dt" [drivers/regulator/mc13892-regulator.ko] undefined!

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>