dts: gen_defines.py: Convert to use f-strings

Use f-strings instead of .format() to makes all the macro identifier and
multiline comment building easier to read.

f-strings were added in Python 3.6, which is required by Zephyr now.

Convert some

    ... + "_" + ...

string building to f-strings as well.

f-strings are a bit faster too (because they avoid format()/str()
lookups), though it's not likely to be noticeable here.

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