scripts: edtlib: Make _binding_include() global to fix pylint warning

Doesn't use 'self'. Fixes this pylint warning:

    scripts/dts/edtlib.py:272:4: R0201: Method could be a function
    (no-self-use)

Fixing pylint warnings for a CI check.

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