[ARM] arch-ixp4xx/io.h: make const args const to remove compiler warning

Compiler warning fix; the inline callers of these APIs were changed
to have const vaddr parameters.

Signed-off-by: John Bowler <jbowler@acm.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>