scripts/dts: Remove confusing deepcopy() in generate_node_defines()
scripts/dts: Remove confusing deepcopy() in generate_node_defines()

After lots of reverse-engineering, I understand why deepcopy() is used:

 1) Various functions defined in scripts/dts/extract/* modify the list
    of names in-place

 2) A plain list.copy() won't work when the *-names property happens to
    have a single name in it, because devicetree.py gives a string
    instead of a list in that case

Using deepcopy() to solve (2) is very confusing (especially with no
comments), because no deep copying is actually needed.

Get rid of deepcopy(), add a helper function for fetching the names, and
some comments.

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,ldts/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,Vudentz/zephyr,ldts/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,ldts/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr