ARM: imx: refactor the io mapping macro

This makes it more assembler friendly and allows it to be used in situation
that need an unsigned long and not a pointer.  Also the naming is
clearer.  IOMEM is introduced without IMX_ prefix as it is used this way
in more than one ARM subarch and it might become globally available
soon.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>