Patch from Deepak Saxena

Mark the ioremap'd cookie/pointer in said functions as const since
we should not be actualy touching the data. This fixes a slew of
compile warnings on IXP4xx as our reads[bwl] already mark this
parameter as const.
Signed-off-by: Deepak Saxena <dsaxena@plexity.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>