ARM: dts: bcm2711: force CMA into first GB of memory

arm64 places the CMA in ZONE_DMA32, which is not good enough for the
Raspberry Pi 4 since it contains peripherals that can only address the
first GB of memory. Explicitly place the CMA into that area.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Acked-by: Stefan Wahren <wahrenst@gmx.net>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>