mtd: fix scb2_flash build when PCI is not enabled

scb2_flash.c is a PCI driver, but its build fails when CONFIG_PCI
is not enabled, so make it depend on PCI.

drivers/mtd/maps/scb2_flash.c:237:1: warning: data definition has no type or storage class
drivers/mtd/maps/scb2_flash.c:237:1: warning: type defaults to 'int' in declaration of 'module_pci_driver'
drivers/mtd/maps/scb2_flash.c:237:1: warning: parameter names (without types) in function declaration

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>