opti621: add PIO 4 support

* Add PIO 4 support.

While at it:

* Use a single struct ide_port_info instance for OPTi621 and OPTi621X.

Based on a bugreport from Juergen Kosel & inspired by pata_opti.c code.

Tested-by: Juergen Kosel <juergen.kosel@gmx.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>