[PowerPC] Fix scc z8530 driver

Parts of the z8530 driver were still using the SUN channel spacing.

This was invalid on PowerMac and QEMU, where the attachment was to escc,
not escc-legacy. This means the driver has apparently NEVER worked properly
on Macintosh hardware.

Add documentation for the channel spacing details, and change to using
driver-specific initialization instead of hardcoded spacing so either
spacing can be used.

Fixes boot hang in QEMU when using the serial console, and fixes use on
Xserve serial (and presumably PowerMacs that have a Stealth Serial port
or similar)

Reviewed by: jhibbits
Sponsored by: Tag1 Consulting, Inc.
Differential Revision: https://reviews.freebsd.org/D24661