doc: board: raspberrypi: Add documentation

Add documentation about the different configuration files for the
RaspberryPi board family.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
3 files changed