arm64: dts: broadcom: Add reference to Raspberry Pi 3 B+

This adds a reference to the dts of the Raspberry Pi 3 B+
in arm, so don't need to maintain the content in arm64.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Eric Anholt <eric@anholt.net>