TXx9: Add TX4938 ATA support (v3)

Add a helper routine to register tx4938ide driver and use it on
RBTX4938 board.

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