greybus: Documentation: Document Authentication interfaces

This patch defined userspace interface of the CAP protocol.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Jun Li <li_jun@projectara.com>
Tested-by: Jun Li <li_jun@projectara.com>
Signed-off-by: Alex Elder <elder@linaro.org>