powerpc: Fix programmer's switch driver and add to GENERIC

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.

Reviewed by: jhibbits (approved w/ removal of unused sc_dev var)
Sponsored by: Tag1 Consulting, Inc.
Differential Revision: https://reviews.freebsd.org/D29131