ARM: dts: Add model-specific compatible strings

The upstream Pi DTs are model-specific, with both the model and
compatible strings identifying the model. Downstream groups the
closely-related models, with only the chip name in the compatible
strings and a model string patched by the firmware.

Bring the downstream model-specific DTs closer to upstream by
adding model-specific compatible strings.

See: https://github.com/raspberrypi/firmware/issues/943



Signed-off-by: Phil Elwell <phil@raspberrypi.org>