mips: add asm/mipsmtregs.h for MIPS multi-threading

To be compatible with old u-boot used by lots of MT7621 devices, the u-boot
needs to boot-up MT7621's all cores, and all VPES of each core.

This patch adds asm/mipsmtregs.h from linux kernel which is need for
boot-up VPEs.

Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>