[OS-249] Add new pi3 revision code

[Ealdwulf]

This PR adds the new pi3 resvision code reported in #294
We also need to add the 3B+, but lets do that once we know everything that needs doing for that.
OS-249

See https://www.raspberrypi.org/documentation/hardware/raspberrypi/revision-codes/README.md

[tombettany]

Can you create a JIRA ticket to link with this so we have a record of whether it has been QA'd. Also, can you create another ticket to revisit how we do this detection because this highlights how fragile this whole mechanism is.

[radujipa]

Checked on the official RPi docs, looks good

Also, I started the 3B+ branch here a while back 5423ca1