MIPS: Loongson64: Reserve vgabios memory on boot

vgabios is passed from firmware to kernel on Loongson64 systems.
Sane firmware will keep this pointer in reserved memory space
passed from the firmware but insane firmware keeps it in low
memory before kernel entry that is not reserved.

Previously kernel won't try to allocate memory from low memory
before kernel entry on boot, but after converting to memblock
it will do that.

Fix by resversing those memory on early boot.

Cc: stable@vger.kernel.org
Fixes: a94e4f24

 ("MIPS: init: Drop boot_mem_map")
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>