MIPS: TXx9: Add RBTX4939 board support

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

 create mode 100644 arch/mips/txx9/rbtx4939/Makefile
 create mode 100644 arch/mips/txx9/rbtx4939/irq.c
 create mode 100644 arch/mips/txx9/rbtx4939/prom.c
 create mode 100644 arch/mips/txx9/rbtx4939/setup.c
 create mode 100644 include/asm-mips/txx9/rbtx4939.h