ARM: dts: Remove CMA allocation from Pi 4 dts

The 5.5 tree includes a patch to disable the CMA command line
parameter and replace it with properties from a DT node.
The upstream Pi 4 .dts, now used downstream with modifications,
includes the "linux,cma" node, but only reserves 32MB which is
often not enough.

Temporarily remove the "linux,cma" node to reenable the command line
parameter.

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