ARM: footbridge: fix build with PCI disabled

The dc21285.c source file cannot be built when CONFIG_PCI is
disabled, because it calls a number of PCI core interfaces.

This changes the Makefile so we don't include this file in the
build if CONFIG_PCI is disabled. No other code references anything
defined inside of this file in this case.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <linux@arm.linux.org.uk>