[MIPS] TXx9: Reorganize code

Move arch/mips/{jmr3927,tx4927,tx4938} into arch/mips/txx9/ tree.
This will help more code sharing and maintainance.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>