When the BL31 for the Allwinner H616 runs in DRAM, we need to make sure
we tell the non-secure world about the memory region it uses.

Add a reserved-memory node to the DT, which covers the area that BL31
could occupy. The "no-map" property will prevent OSes from mapping
the area, so there would be no speculative accesses.

Change-Id: I808f3e1a8089da53bbe4fc6435a808e9159831e1
Signed-off-by: Andre Przywara <andre.przywara@arm.com>