m68k/mac: Disentangle VIA/RBV and NuBus initialization

m68k/mac: Disentangle VIA/RBV and NuBus initialization

The Nubus subsystem should not be concerned with differences between VIA,
RBV and OSS platforms. It should be portable across Macs and PowerMacs.
This goal has implications for the initialization code relating to bus
locking and slot interrupts.

During Nubus initialization, bus transactions are "unlocked": on VIA2 and
RBV machines, via_nubus_init() sets a bit in the via2[gBufB] register to
allow bus-mastering Nubus cards to arbitrate for the bus. This happens
upon subsys_initcall(nubus_init). But because nubus_init() has no effect
on card state, this sequence is arbitrary.

Moreover, when Penguin is used to boot Linux, the bus is already unlocked
when Linux starts. On OSS machines there's no attempt to unlock Nubus
transactions at all. (Maybe there's no benefit on that platform or maybe
no-one knows how.)

All of this demonstrates that there's no benefit in locking out
bus-mastering cards, as yet. (If the need arises, we could lock the bus
for the duration of a timing-critical operation.) NetBSD unlocks the
Nubus early (at VIA initialization) and we can do the same.

via_nubus_init() is also responsible for some VIA interrupt setup that
should happen earlier than subsys_initcall(nubus_init). And actually, the
Nubus subsystem need not be involved with slot interrupts: SLOT2IRQ
works fine because Nubus slot IRQs are geographically assigned
(regardless of platform).

For certain platforms with PDS slots, some Nubus IRQs may be platform
IRQs and this is not something that the NuBus subsystem should worry
about. So let's invoke via_nubus_init() earlier and make the platform
responsible for bus unlocking and interrupt setup instead of the NuBus
subsystem.

Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>