[MIPS] TXx9: Cleanups for 64-bit support

* Unify (and fix) mem_tx4938.c and mem_tx4927.c
* Simplify prom_init
* Kill volatiles and unused definitions for tx4927.h and tx4938.h

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