staging: kpc2000: Add dependency on MFD_CORE to kconfig symbol 'KPC2000'

The kpc2000 core makes calls against functions conditionally exported
upon selection of the kconfig symbol MFD_CORE. Therefore, the kpc2000
core depends upon the mfd_core, and that dependency must be tracked in
Kconfig to avoid potential build issues.

Signed-off-by: Geordan Neukum <gneukum1@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>