[PATCH] mips64: Retrieve MAX_PHYSMEM_BITS from vmcoreinfo

Retrieve MAX_PHYSMEM_BITS from vmcoreinfo for mips64, which was added
by kernel commit 1d50e5d0c505 ("crash_core, vmcoreinfo: Append
'MAX_PHYSMEM_BITS' to vmcoreinfo").  This makes makedumpfile adaptable
for future MAX_PHYSMEM_BITS changes.

Also ensure backward compatibility for kernel versions in which
MAX_PHYSMEM_BITS is not available in vmcoreinfo.

Signed-off-by: Youling Tang <tangyouling@loongson.cn>