hda: add macbook air 6,1 and 6,2 GPIO to enable amplifier

GPIO0 needs to be flipped on to enable the amplifier.

The CS4208 looks /sort/ of like previous cirrus logic codecs,
enough for audio to work. This actually provides working audio
on my mbair 6,2 but it doesn't yet detect the jack.

That requires patches to the pin configs, which is going to
need some further work.

Differential Revision: https://reviews.freebsd.org/D49350
Reviewed by: christos