scripts: dts: Fix misc. pylint warnings in old scripts

Fix pylint warnings for bad indent, redundant len()s in conditionals,
tests that could be improved with 'in', methods that don't use 'self',
and type()s where isinstance() is more common.

Preparation for adding a CI check.

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