pmdomain: renesas: rcar-sysc: Add genpd OF provider at 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 split the initialization into two steps. The first
part keep registering the PM domains with genpd at early_initcall, as this
is needed to bringup the CPUs for R-Car H1, by calling
rcar_sysc_power_up_cpu(). The second and new part, moves the registration
of the genpd OF provider 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/20250701114733.636510-2-ulf.hansson@linaro.org