scripts: dtlib: Suppress _init_tokens() pylint warning

Suppress this pylint warning so that it can be enabled in the upcoming
CI check. The code is safe.

    scripts/dts/dtlib.py:1904:13: W0631: Using possibly undefined loop
    variable 'i' (undefined-loop-variable)

Also add some more comments to clarify _init_tokens().

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