boards/intel_adsp_cavs15: Update docs to cover twister integration

Documentation update with information on how to run twiter integration
testing on this board.  Also elaborate the discussion of permissions
setup.

(Longer term, it would be better to include a udev example of how to
set permissions on those files rather than doing it manually, and to
include fuller instructions on how to build the needed SOF driver
instead of just priving a link to a github tree.)

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>