of/fdt: skip unflattening of disabled nodes

For static DT usecases, we don't need the disabled nodes and can skip
unflattening. This saves a significant amount of RAM in memory constrained
cases. In one example on STM32F469, the RAM usage goes from 118K to 26K.

There are a few cases in the kernel that modify the status property
dynamically. These all are changes from enabled to disabled, depend on
OF_DYNAMIC or are not FDT based (PDT based).
Tested-by: NNicolas Pitre <nico@linaro.org>
Reviewed-by: NFrank Rowand <frowand.list@gmail.com>
Signed-off-by: NRob Herring <robh@kernel.org>

drivers/of/fdt.c

@@ -396,6 +396,10 @@ static int unflatten_dt_nodes(const void *blob,
 		if (WARN_ON_ONCE(depth >= FDT_MAX_DEPTH))
 			continue;
 
+		if (!IS_ENABLED(CONFIG_OF_KOBJ) &&
+		    !of_fdt_device_is_available(blob, offset))
+			continue;
+
 		if (!populate_node(blob, offset, &mem, nps[depth],
 				   &nps[depth+1], dryrun))
 			return mem - base;