greybus: hd: add bus_id attribute

The greybus host device id can only be read by parsing the uevent if one
wants to identify a specific host device 'or' bus. This is 'lsgb' uses
today.

This change adds a bus_id attribute so libraries can identify multiple
host devices 'or' bus if they exist.

Testing Done:
Tested on Arche,
'cat /sys/bus/greybus/devices/greysbus1/bus_id'

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>