dtc: add ability to make nodes conditional on them being referenced

This is needed when importing mainline DTs into U-Boot, as some started
using this /omit-if-no-ref/ tag, so won't compile with U-Boot's current
dtc copy. This is just a cherry-pick of the patch introducing this
feature.
Original commit message from Maxime:
------------------
A number of platforms have a need to reduce the number of DT nodes,
mostly because of two similar constraints: the size of the DT blob, and
the time it takes to parse it.

As the DT is used in more and more SoCs, and by more projects, some
constraints start to appear in bootloaders running from SRAM with an
order of magnitude of 10kB. A typical DT is in the same order of
magnitude, so any effort to reduce the blob size is welcome in such an
environment.

Some platforms also want to reach very fast boot time, and the time it
takes to parse a typical DT starts to be noticeable.

Both of these issues can be mitigated by reducing the number of nodes in
the DT. The biggest provider of nodes is usually the pin controller and
its subnodes, usually one for each valid pin configuration in a given
SoC.

Obviously, a single, fixed, set of these nodes will be used by a given
board, so we can introduce a node property that will tell the DT
compiler to drop the nodes when they are not referenced in the tree, and
as such wouldn't be useful in the targetted system.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>