1) The E1 seems to have the int. siop wired to irq 14-level and the internal
wdc wired to irq 14-edge. special case and fail the wdc probe on E1's.
2) If we fail to map the NVRAM registers, return, rather than trying to talk
to them and panic'ing the box.
3) revert my previous "fix" to pnpbus to make irq's default to level. It
was wrong, and didn't even fix the powerstack.
With this, we have limited PowerStack E1 support. The machine cannot
talk to it's IDE controller, and cannot detect it's boot device
automatically, but it does come up and run. Tested with NFS root.
64b69ee83d