bus: fsl-mc: add missing __iomem attribute

This pointer to an i/o register block is missing the __iomem attribute,
so add it. The issue was found by 0-day sparse run.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Laurentiu Tudor <laurentiu.tudor@nxp.com>
Link: https://lore.kernel.org/r/20201216161015.29060-1-laurentiu.tudor@nxp.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>