MIPS: Do not request resources for crashkernel if one isn't defined

When KEXEC is enabled but crashkernel details are not passed through the
kernel commandline unnecessary resources are requested (start==end==0)

Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14607/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>