GPIO_RDC321X: select MFD_SUPPORT to squelch kconfig warning

Fix kconfig dependency warning to satisfy dependencies:

warning: (GPIO_RDC321X && PCI && GPIOLIB) selects MFD_RDC321X which has unmet direct dependencies (MFD_SUPPORT && PCI)

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>