ARM64: dts: Add Zero 2 W and CM4S

Zero 2 W and CM4S are 64-bit-capable, so their DTBs should be buildable
in the arm64 tree.

See (misplaced) comment in https://github.com/raspberrypi/linux/issues/4857



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