ARM: dts: meson6: add reserved memory zone for the HW ROM firmware - WiP

The Amlogic vendor kernel reserved 1MiB of memory between 511 and
512MiB. This can be seen in the boot-logs of the 3.10 vendor kernel. All
of these print the following message during boot:
reserved 511M-512M 1M memory for firmware

Reserve this memory to prevent devices from haning randomly.

WiP - not tested yet!

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>