gpio: rdc321x needs to select MFD_CORE

Fix rdc321x-southbridge build:  GPIO_RDC321X needs to select
MFD_CORE so that the core is built at the same (or higher)
tristate level.

rdc321x-southbridge.c:(.devinit.text+0x6103): undefined reference to `mfd_add_devices'
rdc321x-southbridge.c:(.devexit.text+0xe5f): undefined reference to `mfd_remove_devices'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc:	Florian Fainelli <florian@openwrt.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>