[MIPS] TXx9: Add 64-bit support

SYS_SUPPORTS_64BIT_KERNEL is enabled for RBTX4927/RBTX4938, but
actually it was broken for long time (or from the beginning).  Now it
should work.

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