misc: introduce new mos6522 VIA device and enable it for ppc builds

The MOS6522 VIA forms the bridge part of several Mac devices, including the
Mac via-cuda and via-pmu devices. Introduce a standard mos6522 device that
can be shared amongst multiple implementations.

This is effectively taking the 6522 parts out of cuda.c and turning them
into a separate device whilst also applying some style tidy-ups and including
a conversion to trace-events.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>