cfattach code for TC SCSI option cards and ioasic 53c94 baseboard SCSI.
ascvar.h: shared softc declarations
asc_ioasic.c: ioasic front-end code.
asc_tc.c: Turbochannel option (and 5000/200 basebard) front-end code.
* ioasic_attach meeds more work to eliminate pmax_type dependency
and to verify the clocks speed passed to 53c94.
* Add prototypes for asc script entry points; should compile with
-Wstrict-prototypes -Wmissing-prototypes.
* Use tcvar.h interface. The usage of tc_syncbus() and tc_mb() may
not be quite stylistically on an Alhpa, but it apparently makes no
difference on the eerly-generation Alpha CPUs in TC Alphas.
