[ARM] fix bogus cast in IXP2000 I/O macro

Physical addresses are not valid pointers of any sort and should
not be cast to such.

Signed-off-by: Deepak Saxena <dsaxena@plexity.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>