dtlib: use f-strings where it makes sense

The library was originally developed before Python 3.6 was the minimum
supported version. Use f-strings now that we can do that, as they tend
to be easier to read.

There are a few places where str.format() makes sense to preserve,
specifically where the same argument is used multiple times; leave
those alone.

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