docs: Update DTB format references

There were two references to devicetree/booting-without-of.rst (which has
been removed) for DTB format information, and
devicetree/usage-model.rst pointed to
https://elinux.org/Device_Tree_Usage. Change all three of these references to
https://www.devicetree.org/specifications/.

Signed-off-by: Milan Lakhani <milan.lakhani@codethink.co.uk>
Link: https://lore.kernel.org/r/1611149511-4990-1-git-send-email-milan.lakhani@codethink.co.uk
Signed-off-by: Jonathan Corbet <corbet@lwn.net>