kexec: mips: Fix mem parameters

"mem=" is useful to indicate the memory region when capture kernel boot.
Otherwise, capture kernel will breakdown the memory of panic kernel.
Although it can be add by user, adding "mem" by software is a better way.
What's more, "mem" should contain elfcorehdr range. Elfcorehdr memory
should be managed by kernel.

Fixes: 7bd25165 ("kexec-tools: mips: Remove commandline parameter "mem"")
Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Simon Horman <horms@verge.net.au>