cmake: CMake devicetree related linker functions implemented

This commit introduces zephyr_linker_dts CMake functions for creation of
linker scripts based on devicetree nodes.

The linker devicetree functions supports the following features:
- Configuration of memory sections based on devicetree nodes

Overview of functions introduced with this commit:
- zephyr_linker_dts_memory

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>