dts: handle unspecified phandle-array elements

The DTS language permits zeroing out phandles in a phandle array to
say "there's nothing at this index", and dtlib manages that correctly,
but edtlib and gen_defines.py aren't equipped to do so.

Fix this by allowing None elements in the lists of ControllerAndData
values returned by edtlib for such properties.

Handle that in gen_defines.py by setting the generated
DT_N_<node>_P_<prop>_IDX_<i>_EXISTS macro to 0 in such cases.

The DT_N_<node>_P_<prop>_LEN macro still accounts for the entire
length of the phandle-array; it's just that some indexes may be
missing data.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>