[PowerPC] Fix programmer's switch driver and add to GENERIC

[bdragon]

Summary

Older G4 and G3 models have a programmer's switch that can be used to
generate an interrupt to drop into the debugger.

This code hadn't been tested for a long time. It had been broken back
in 2005 in r153050.

Repair and modernize the code and add it to GENERIC.

[jhibbits]

sys/powerpc/powermac/pswitch.c

50–52

do you need sc_dev? I don't see you using it anywhere.