dm: Update README.txt to clarify device tree usage

In the section about Device Trees add a paragraph at the end that
clarifies how we decide of a tree is valid or not.  We say that all
bindings must either be in the specification (link provided) or in our
device-tree-bindings directory.  We say that most of these come from the
Linux Kernel and as such some design decisions are made for us already,
but that in most cases we wish to retain compatibility.

Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>