ARM: dts: Add model-specific compatible strings

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>
(cherry picked from commit d90929181836e8a049275d8407cff70fb93b7f17
https://github.com/raspberrypi/linux raspi/rpi-4.15.y)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>