greybus: gpbridge: Expose protocol_id in sysfs

Right now, userspace doesn't have any way to find what protocol does a
gpbridge device implement. And this is essential for the scripts to
know, to expect what kind of device will be present inside the gpbN
directory.

Expose 'protocol_id' in sysfs to fix that.

Tested by checking that the field appears with GP module on EVT 1.5.

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