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>

doc/driver-model/README.txt

@@ -449,6 +449,15 @@ The driver model tree is intended to mirror that of the device tree. The
 root driver is at device tree offset 0 (the root node, '/'), and its
 children are the children of the root node.
 
+In order for a device tree to be valid, the content must be correct with
+respect to either device tree specification
+(https://www.devicetree.org/specifications/) or the device tree bindings that
+are found in the doc/device-tree-bindings directory.  When not U-Boot specific
+the bindings in this directory tend to come from the Linux Kernel.  As such
+certain design decisions may have been made already for us in terms of how
+specific devices are described and bound.  In most circumstances we wish to
+retain compatibility without additional changes being made to the device tree
+source files.
 
 Declaring Uclasses
 ------------------