greybus: Documentation/sysfs: add entry for host device bus_id

Add description for greybus host device bus_id attribute

Signed-off-by: Sandeep Patil <sspatil@google.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>