RPI: Add entry for Raspberry Pi 4 model B

The Raspebrry Pi 4 uses the new revision code scheme as documented by
the foundation. This change adds an entry for this board as well.

Signed-off-by: Andrei Gherzan <andrei@balena.io>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>