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>

arch/arm/boot/dts/overlays/vc4-fkms-v3d-pi4-overlay.dts

@@ -8,7 +8,7 @@
 #include "cma-overlay.dts"
 
 &frag0 {
-       size = <(512*1024*1024)>;
+	size = <((320-4)*1024*1024)>;
 };
 
 / {

arch/arm/boot/dts/overlays/vc4-kms-v3d-pi4-overlay.dts

@@ -10,7 +10,7 @@
 #include "cma-overlay.dts"
 
 &frag0 {
-       size = <(512*1024*1024)>;
+	size = <((320-4)*1024*1024)>;
 };
 
 / {