devicetree: Fix DT_PROP/DT_PROP_BY_IDX for phandle(s)

DT_PROP for a phandle property should return the node that phandle
points to (similar for DT_PROP_BY_IDX for phandles)  and this wasn't
working as the define generator didn't create the proper defines for
phandle(s).

Fix the generator and add some tests to make sure this continues to
work correctly.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>