cmake: scripts: support SoC extension

Fixes: #72374

Support extending an existing SoC with new CPU clusters.

This commit introduces the following changes to allow an SoC to be
extended out-of-tree.

The SoC yaml schema is extended to support an extend field which
will be used to identify the SoC to be extended with extra CPU clusters.

A SoC 'a_soc' can be extended like this:
> socs:
>   extend: a_soc
>   cpuclusters:
>     - name: extra_core

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>