pmdomain: renesas: rcar-sysc: Make rcar_sysc_onecell_np __initdata

rcar_sysc_onecell_np() is only used by functions marked __init, so it
can be freed when init memory is freed.

Fixes: c5ae5a0c ("pmdomain: renesas: rcar-sysc: Add genpd OF provider at postcore_initcall")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/e20a848ff952924f8f58c335f9a0242cb2565921.1755090234.git.geert+renesas@glider.be


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>