Added Mixer8 per MichaelMeissner feature request

[chipaudette]

Adding an 8 channel mixer.

This is per the feature request from MichaelMeissner in the forum post:

https://forum.pjrc.com/threads/61062-Updating-Design-Tool-for-Teensy-4-0-amp-4-1?p=240913&viewfull=1#post240913

Code is based directly on Mixer4 and extended to 8 channels.