overlays: Add 'upstream' overlay

The 'upstream' overlay makes the necessary changes for a downstream
.dtb to be used with an upstream kernel. It is currently made up from
three other overlays - vc4-kms-v3d, dwc2 and upstream-aux-interrupt.
The VPU firmware will soon be made to automatically load this overlay
when an upstream kernel is detected (using the trailer supplied by the
mkknlimg script).

See: https://github.com/raspberrypi/linux/pull/2393



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