Add support for the PC98 platform to the ATA driver.

Add support for the PC98 platform to the ATA driver.
This mostly consists of functionality to serialize accesses to
the two ATA channels (which can also be used to "fix" certain
PCI based controllers).
Add support for Acard controllers.
Enable the ATA driver in PC98 GENERIC, and add device hints.
Update man page with latest support.

The PC98 core team has kindly provided me with a PC98
machine that made this all possible, thanks to all that
contributed to that effort, without that this would
probably newer have been possible..

Approved by: re@