pmdomain: renesas: rmobile-sysc: Move init to postcore_initcall

Subsequent changes to genpd adds a limitation that registering a genpd OF
providers must be done after its bus registration, which is at
core_initcall. To adopt to this, let's move to a postcore_initcall.

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20250701114733.636510-3-ulf.hansson@linaro.org