brcm2708: add compatible strings

brcm2708: add compatible strings

This patch adds the compatible string for the various RPIs from
4.14 upstream.

Note: The 4.14 upstream does not include the compute modules.
If the CM* would just house the SoC, it could in theory use the
"raw" chip compatible string. However, these CM boards also come
with RAM and eMMC. So they have to have a proper comaptible.

For now, "raspberrypi,compute-module-{1|3}" will be good enough.

Note2: The original CM was renamed to CM1 when CM3 was released.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>