[MIPS] rbtx4938: misc cleanups

* Do not use non-standard I/O accessors, such as reg_rd08, etc.
* Kill unnecessary wbflush()
* Kill tx4938_mips.h
* Kill unnecessary includes

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