rpi4: Reserve resident BL31 region from non-secure world

The GPU firmware loads the armstub8.bin (BL31) image at address 0, the
beginning of DRAM. As this holds the resident PSCI code and the SMP
pens, the non-secure world should better know about this, to avoid
accessing memory owned by TF-A. This is particularly criticial as the
Raspberry Pi 4 does not feature a secure memory controller, so
overwriting code is a very real danger.

Use the newly introduced function to add a node into reserved-memory
node, where non-secure world can check for regions to be excluded from
its mappings.

Reserve the first 512KB of memory for now. We can refine this later if
need be.

Change-Id: I00e55e70c5c02615320d79ff35bc32b805d30770
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

plat/rpi/rpi4/rpi4_bl31_setup.c

@@ -245,6 +245,10 @@ static void rpi4_prepare_dtb(void)
 		return;
 	}
 
+	/* Reserve memory used by Trusted Firmware. */
+	if (fdt_add_reserved_memory(dtb, "atf@0", 0, 0x80000))
+		WARN("Failed to add reserved memory nodes to DT.\n");
+
 	ret = fdt_pack(dtb);
 	if (ret < 0)
 		ERROR("Failed to pack Device Tree at %p: error %d\n", dtb, ret);