Allow querying of the firmware revision of the device

example:

4.10

Tested on ZII RDU2 platform and on Intel x86_64 PC.
Signed-off-by: NNick Dyer <nick@shmanahar.org>
Tested-by: NNikita Yushchenko <nikita.yoush@cogentembedded.com>
Signed-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>