openrisc: io: Add missing __iomem annotation to iounmap()

With C=1:

    drivers/soc/renesas/rmobile-sysc.c:330:33: sparse: sparse: incorrect type in argument 1 (different address spaces) @@     expected void *addr @@     got void [noderef] __iomem *[assigned] base @@
    drivers/soc/renesas/rmobile-sysc.c:330:33: sparse:     expected void *addr
    drivers/soc/renesas/rmobile-sysc.c:330:33: sparse:     got void [noderef] __iomem *[assigned] base

Fix this by adding the missing __iomem annotation to iounmap().

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Stafford Horne <shorne@gmail.com>