ARM: bcm2835: Add devicetree for the Raspberry Pi 3.

While this devicetree also works for booting in 32-bit mode, it's
placed in arm64 since it's a 64-bit CPU (as suggested by Arnd).

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Stephen Warren <swarren@wwwdotorg.org> (v1)
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>