gen_defines: play tricks to benefit DT_INST users

Even though it is about to be done for sound technical reasons, a
subsequent patch adding access to all device nodes at the last minute
in the 2.3 release is going to be playing a bit of a fast one on
the Zephyr community, especially users of DT_INST APIs.

In particular, instance numbers are currently allocated only to
enabled nodes, but that will not be true soon: *every* node of a
compatible will be allocated an instance number, even disabled ones.

This is especially unfortunate for drivers and applications that
expect singletons of their compatibles, and use DT_INST(0, ...) to
mean "the one enabled instance of my compatible".

To avoid gratuitous breakage, let's prepare for that by sorting each
edt.compat2nodes sub-list so that enabled instances always come before
disabled ones.

This doesn't break any API guarantees, because there basically *are*
no ordering guarantees, in part precisely to give us the flexibility
to do things like this. And it does help patterns that use instances 0
through N-1, including the important singleton case.

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