mips: Make sure kernel memory is in iomem

Kernel memory isn't necessarily added to the memory tables, so it
wouldn't show up in /proc/iomem.  This was breaking kdump, which
requires these memory addresses to work correctly.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Acked-by: David Daney <ddaney@caviumnetworks.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Patchwork: http://patchwork.linux-mips.org/patch/4937/