dts: edtlib: Add a Property.type field

Deriving the type from looking at Property.val gets awkward e.g. when
there are many types that make Property.val a list. Instead, save the
type as given in the binding in Property.type.

Let Property.type just be a string. This has typo potential, but is nice
and flexible (and easy to print), and errors will probably be pretty
obvious.

Show the type in Property.__repr__() as well. This automatically gives
some test coverage.

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