Parts of the z8530 driver were still using the SUN channel spacing.
This was wildly breaking QEMU powerpc32 (it would have a constant interrupt on channel A, because the driver would read from / write to the wrong location, as QEMU only has the sun spacing for escc-legacy attachment)
Fix the channel spacing in the ic header, and document the different spacing.
With this patch, the ppc32 QEMU serial console attaches properly and does not hang. Presumably also fixes attaching on a rackmac (which I will test later)