clk: renesas: mstp: Add genpd OF provider at postcore_initcall()

Genpd OF providers must now be registered after genpd bus registration.
However, cpg_mstp_add_clk_domain() is only called from CLK_OF_DECLARE(),
which is too early.  Hence on R-Car M1A, R-Car H1, and RZ/A1, the
CPG/MSTP Clock Domain fails to register, and any devices residing in
that clock domain fail to probe.

Fix this by splitting initialization into two steps:
  - The first part keeps on registering the PM domain with genpd at
    CLK_OF_DECLARE(),
  - The second and new part moves the registration of the genpd OF
    provider to a postcore_initcall().

See also commit c5ae5a0c ("pmdomain: renesas: rcar-sysc: Add
genpd OF provider at postcore_initcall").

Fixes: 18a3a510 ("pmdomain: core: Add the genpd->dev to the genpd provider bus")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/81ef5f8d5d31374b7852b05453c52d2f735062a2.1755073087.git.geert+renesas@glider.be