The same SoC may have different clocks and/or module clock parents,
depending on SoC revision.  One option is to use different sets of clock
tables for each SoC revision.  However, if the differences are small, it
is much more space-efficient to have a single set of clock tables, and
fix those up at runtime instead.

Hence provide three helpers:
  - Two helpers to NULLify core and module clocks that do not exist on
    some revisions (NULLified clocks are skipped during the registration
    phase),
  - One helper to reparent module clocks that have different clock
    parents.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>