Enable via_alt_mapping on the Performa 588 and tidy up related documentation.

I'm betting that remapped IRQs work just fine on the Performa 580 series since it works on the LC 630 and the logic board part numbers are reputedly the same.

And the consensus seems to be that the Mac TV is essentially a Performa 550, not dissimilar to the Performa 520, so set the via_type accordingly.
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>