Add support for SQ66

[ed-xmos]

@tomblackie-xmos has expressed great interest in getting a port in this lib for https://www.xmos.com/xk-voice-sq66

Probably a couple of days work

[tomblackie-xmos]

As background - The XMOS sales team (US and EU in particular) have standardised on the -SQ66 board for customer demo systems, and they have provided our distributors with these boards. As a result, the ability to re-target some recent app notes that use the lib_board_support to run on this -SQ66 board is highly desirable.