cmake: dts: import devicetree symbols into CMake

This commit introduces devicetree API in CMake so that devicetree
properties and register block can be used in the CMake build system.

The script scripts/dts/gen_dts_cmake.py processes the edt.pickle and
generates a corresponding devicetree property map in a devicetree_target
that is then used in CMake.

The following devicetree API has been made available in Zephyr CMake:
- dt_nodelabel(<var> NODELABEL <label>)
- dt_node_exists(<var> PATH <path>)
- dt_node_has_status(<var> PATH <path> STATUS <status>)
- dt_prop(<var> PATH <path> PROPERTY <prop>)
- dt_prop(<var> PATH <path> INDEX <idx> PROPERTY <prop>)
- dt_num_regs(<var> PATH <path>)
- dt_reg_addr(<var> PATH <path> [INDEX <idx>])
- dt_reg_size(<var> PATH <path> [INDEX <idx>])
- dt_has_chosen(<var> PROPERTY <prop>)
- dt_chosen(<var> PROPERTY <prop>)

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