rpi: link to another model number info source

This source has been blessed by Dom Cobley at the RPi Foundation, so seems
like the best source to refer to. It's a superset of and consistent with
the other sources.

Cc: Lubomir Rintel <lkundrak@v3.sk>
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>