fdt: prevent clearing memory node if there are no banks

Avoid clearing the reg property in the memory DT node if no memory
banks have been specified for a board (CONFIG_NR_DRAM_BANKS == 0).
This allows boards to let U-Boot skip the DT memory tinkering in case
other firmware has already setup the node properly before.
This should be safe as all callers of fdt_fixup_memory_banks that use
a computed <banks> value put at least 1 in there.
Add some documentation comments to the header file.

Signed-off-by: Andre Przywara <osp@andrep.de>
Acked-by: Simon Glass <sjg@chromium.org>

common/fdt_support.c

@@ -454,6 +454,9 @@ int fdt_fixup_memory_banks(void *blob, u64 start[], u64 size[], int banks)
 		return err;
 	}
 
+	if (!banks)
+		return 0;
+
 	len = fdt_pack_reg(blob, tmp, start, size, banks);
 
 	err = fdt_setprop(blob, nodeoffset, "reg", tmp, len);