mips: n64: add __iomem for writel call

mips: n64: add __iomem for writel call

sparse: incorrect type in argument 2 (different address spaces) @@
expected void volatile [noderef] __iomem *mem @@
got unsigned int [usertype] *

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202105261445.AcvPd2EE-lkp@intel.com/
Fixes: baec970aa5ba ("mips: Add N64 machine type")
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>