m68k: Mac nubus IRQ fixes (plan E)

Some Macs lack a slot interrupt enable register. So the existing code makes
disabled and unregistered slot IRQ lines outputs set high. This seems to work
on quadras, but does not work on genuine VIAs (perhaps the card still succeeds
in pulling the line low, or perhaps because this increases the settle time on
the port A input, meaning that the CA1 IRQ could fire before the slot line
reads active).

Because of this, the nubus_active flags were used to mask IRQs, which is
actually worse than the problem it tries to solve. Any interrupt masked by
nubus_active will remain asserted and prevent further transitions on CA1. And
so the nubus gets wedged regardless of hardware (emulated VIA ASIC, real VIA
chip or RBV).

The best solution to this hardware limitation of genuine VIAs is to disable the
umbrella SLOTS IRQ when disabling a slot on those machines. Unfortunately, this
means all slot IRQs get disabled when any slot IRQ is disabled. But it is only
a problem when there's more than 1 device using nubus interrupts.

Another potential problem for genuine VIAs is an unregistered nubus IRQ.
Eventually it will be possible to enable the CA1 interrupt by installing its
handler only _after_ all nubus drivers have loaded but _before_ the kernel
needs them, at which time this last problem can be fixed. For now it can be
worked around:

  - disable MacOS extensions
  - don't boot MacOS (use the Emile bootloader instead)
  - get the bootloaders to disable ROM drivers (Penguin does this for video
    cards already, don't know about Emile)
  - physically remove unsupported cards

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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