mfd: Export pm80x_regmap_config

This fixes below build error when CONFIG_MFD_88PM800=m.

ERROR: "pm80x_regmap_config" [drivers/mfd/88pm800.ko] undefined!
make[1]: *** [__modpost] Error 1
make: *** [modules] Error 2

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>