[gta02-core] Wolfson audio application notes

[Werner Almesberger]

FYI: Joerg found a few application notes that make good reading:

General design considerations (very relevent for gta02-core):
http://www.wolfsonmicro.com/uploads/documents/en/Class D Audio Design Tips indd.pdf

More on capless design (not directly relevant for what we're
doing at the moment, but for future study):
http://www.wolfsonmicro.com/uploads/documents/en/WAN0131.pdf

ALC settings (more a software issue):
http://www.wolfsonmicro.com/uploads/documents/en/WAN0178.pdf

- Werner