Zalon 53c720 support added

Hi,
I have added support for 53c720, currently (ab)using the ncr53c8xx.c driver,
but it may have to split in to a seperate driver later. Tested on a bluefish card; I can run fast and wide, using tagged commands, and copy data between two disks. It crashes after a while, but then if I do the same with either the 53c710 or 53c875 drivers a similar thing happens. I'm busy with other things for the next week or so, but I'll get back to it after that.

To enable it answer 'yes' to CONFIG_SCSI_ZALON.

Richard