greybus: gpbridge: add uevent vars for gpbridge devices

The uevent vars now include module, interface, greybus_id,
bundle id, class and gpbridge device specific properties.

This make it consistent with how we are reporting uevents for
all other greybus devices.

Testing Done:
Tested by reading uevent from gpbridge devices that enumerate
using gpb module

Signed-off-by: Sandeep Patil <sspatil@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>