dm: core: Add definitions for multiple ofnode trees

At present, unless OF_LIVE is enabled, ofnode only supports access to one
device tree, the control FDT. This is because only the node offset is
encoded in ofnode, with the tree being implicit.

This makes ofnode (without OF_LIVE) unsuitable for device tree fixups, as
implemented by ft_board_setup() and other such functions.

To solve this, we can use the top bits of the node offset to hold a tree
ID.

Add the definitions for this.

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