dm: core: Add the ofnode multi-tree implementation

Add the logic to redirect requests for the device tree through a function
which can look up the tree ID. This works by using the top bits of
ofnode.of_offset to encode a tree.

It is assumed that there will only be a few device trees used at runtime,
typically the control FDT (always tree ID 0) and possibly a separate FDT
to be passed the OS.

The maximum number of device trees supported at runtime is 8, with this
implementation. That would use bits 30:28 of the node-offset value,
meaning that the positive offset range is limited to bits 27:0, versus
30:1 with this feature disabled. That still allows a device tree of up
to 256MB, which should be enough for most FITs. Larger ones can be
supported by using external data with the FIT, or by enabling OF_LIVE.

Update the documentation a little and fix up the comment for
ofnode_valid().

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/develop/driver-model/livetree.rst

@@ -250,11 +250,14 @@ a flat tree.
 It would be helpful to use livetree for fixups, since adding a lot of nodes and
 properties would involve less memory copying and be more efficient. As a step
 towards this, an `oftree` type has been introduced. It is normally set to
-oftree_default() but can be set to other values. Eventually this should allow
-the use of FDT fixups using the ofnode interface, instead of the low-level
-libfdt one.
+oftree_default() but can be set to other values using oftree_from_fdt().
+So long as OF_LIVE is disabled, it is possible to do fixups using the ofnode
+interface. The OF_LIVE support required addition of the flattening step at the
+end.
 
-See dm_test_ofnode_root() for some examples.
+See dm_test_ofnode_root() for some examples. The ofnode_path_root() function
+causes a flat device tree to be 'registered' such that it can be used by the
+ofnode interface.
 
 
 Internal implementation