dts: dtlib: Ignore manually specified phandles in type checking

Previously, Property.to_node() would allow assignments like

    x = < 1 >;

as long as 1 happened to be a valid phandle. This was deliberate, but
might hide errors, and would make the planned 'phandles' (list of
phandles) and 'phandle-array' (list of phandles and numbers) types a bit
too similar to 'type: array'.

Change Property.to_node() to only accept

    x = < &foo >;

This is probably all we need, and if you really need to accept manually
specified phandles, it can be worked around in other ways.

Piggyback some consistency nits in error messages from the
Property.to_*() functions.

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