greybus: gpbridge: add gpbridge device type

Right now, there is no way to know the device type of gpbridge
(bridged-phy) devices in userspace. So, add that.

Testing Done:
Tested by reading the 'uevent' for gpb device of gpbridge
module

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