powerpc: Add C2K to configuration

Support for the C2K cPCI Single Board Computer from GEFanuc
(PowerPC MPC7448 with a Marvell MV64460 chipset).
All features of the board are not supported yet, but the board
boots, flash works, all Ethernet ports are working and PCI
devices are all found (USB and SATA on PCI1 do not work yet).

Part 5 of 5: add the Kconfig entry for the C2K board.

Signed-off-by: Remi Machet <rmachet@slac.stanford.edu>
Signed-off-by: Paul Mackerras <paulus@samba.org>