mips: reserve elfcorehdr

/proc/vmcore wasn't showing up in kdump kernels.  It turns that that
for Octeon, the memory used by elfcorehdr wasn't being set aside
properly and it was getting clobbered before /proc/vmcore could get
it.  So reserve the memory if it shows up in a memory area managed
by the kernel.

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/4936/