greybus: add documentation for Endo sysfs files

Update the files documenting Greybus-related sysfs files under
Documentation/ to reflect the addition of the two recently-added
Endo attributes.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>