[ARM] omap: Fix IO_ADDRESS() macros

OMAP1_IO_ADDRESS(), OMAP2_IO_ADDRESS() and IO_ADDRESS() returns cookies
for use with __raw_{read|write}* for accessing registers.  Therefore,
these macros should return (void __iomem *) cookies, not integer values.

Doing this improves typechecking, and means we can find those places
where, eg, DMA controllers are incorrectly given virtual addresses to
DMA to, or physical addresses are thrown through a virtual to physical
address translation.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>