ARM: dts: Add devicetree for the Raspberry Pi 3, for arm32 (v6)

Raspbian and Fedora have decided to support the Pi3 in 32-bit mode for
now, so it's useful to be able to test that mode on an upstream
kernel.  It's also been useful for me to use the same board for 32-bit
and 64-bit development.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Olof Johansson <olof@lixom.net>