doc: release: devicetree notes for v3.1

Some significant highlights include:

- new pinctrl and reset controller bindings
- enhanced ADC and PWM bindings
- zephyr-specific generation of MPU regions from devicetree
- new DT CAN API
- enhanced support for conversion from strings in the DTS
  to tokens in C
- many, many enhancements to existing bindings

Many thanks to everyone who contributed these great changes!

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