ARM: dts: bcm2711-rpi: Set a 1GB ZONE_DMA limit

The arm64 initialisation uses the physical address reachable by all
DMA controllers to set the size of ZONE_DMA. This fails on BCM2711
with the current dts files because the declaration of the I/O space
fools it into thinking the legacy 30-bit DMA channels can see most
of the 32-bit space.

Take advantage of the simple nature of the implementation by adding
a node with a restricted dma-ranges property solely so to act as the
limiting factor in the calculation.

See: https://github.com/raspberrypi/linux/issues/5470



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