MIPS: TXx9: Add TX4939 ATA support (v2)

Add a helper routine to register tx4939ide driver and use it on
RBTX4939 board.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>