doc: devicetree release notes for 2.7

There's enough meat here to split the content up into areas:
devicetree.h itself, Python tooling changes, and bindings changes.

I also reworked the section describing vendor prefix fixes to use a
table, which I find more readable on a second pass through it.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>