dts: Remove support for deprecated DTS binding syntax

We deprecated a number of aspects of the DTS binding syntax in Zephyr
2.1.  Remove the support for the deprecated syntax.  Remove from docs
about the deprecated syntax as well.

Removed reference in release-notes-2.1.rst to legacy_binding_syntax
since that anchor doesn't exist anymore.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>