dts: edtlib: Sanity-check 'ranges' assignment syntax

Require either type TYPE_EMPTY ('ranges;') or TYPE_NUMS
('ranges = < 1 2 ... >;').

Putting the check in _check_dt() means it will run for all nodes,
including nodes without bindings, which is handy.

The _split() function already gives a decent error message if 'ranges'
has unexpected length, so skip checking the length.

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