ARM: dts: bcm2711: Restrict CMA to first 768MB

The downstream 32-bit 2711 kernel configuration enables HIGHMEM for
access to more physical RAM. The HIGHMEM zone starts at 0x30000000
(768MB), and allowing the CMA zone to overlap that area causes a
failure during CMA activation.

Avoid the overlap by limiting CMA to the first 768MB. This is overly
restrictive on a 64-bit kernel, but shouldn't cause any practical
problems.

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