soc: renesas: rcar-sysc: Add support for fixing up power area tables

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

Hence provide a helper to NULLify power areas that do not exist on some
revisions (NULLified power areas are skipped during the registration
phase), and support for an optional initialization callback to e.g. fix
up power area tables.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>