overlays: Reduce Pi 4 vc4 CMA size to 320MB

Reduce the default CMA allocation requested by the vc4-kms-v3d-pi4 and
vc4-fkms-v3d-pi4 overlays to 320MB.

Use magic values of the form (<n>*64 - 4)MB to encode default values
of <n>*64MB, allowing these defaults to be distinguished from values
set explicitly by the user with the usual overlay parameters (e.g.
"cma-384"). Only default values will be capped if the Pi RAM is too
small or the gpu_mem setting too large for it to be viable.

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