ixp4xx: Declare MODULE_FIRMWARE usage

Cc: Krzysztof Halasa <khc@pm.waw.pl>
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Krzysztof Hałasa <khc@pm.waw.pl>