MIPS: Return -EINVAL if mem parameter is empty in early_parse_mem()

In the current code, the users usually need to make sure the value
of mem parameter is correct, but it is better to do some check to
avoid potential boot hangs.

This commit checks whether mem parameter is empty, if yes, return
-EINVAL before call memblock_remove() and memblock_add().

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>