[ARM] 4296/1: ixp4xx: compile fix

Fix compilation fail for ixp4xx platforms for the case when CONFIG_IXP4XX_INDIRECT_PCI is set. That is due to the check_signature() is appeared in include/linux/io.h.

Signed-off-by: Vladimir Barinov <vbarinov@ru.mvista.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>