overlays: Set CMA to 512MB on Pi 4 for vc4

Pi 4s have at least 1GB, and there are advantages to having more CMA
available (HEVC works out of the box, support for more complex video
setups, etc.) without significant disadvantages.

Can be overridden by appending a parameter to the dtoverlay line, e.g.
dtoverlay=vc4-fkms-v3d,cma-256

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