ARM: shmobile: rcar-gen2: Remove unneeded once handling

rcar_gen2_pm_init() is only called from the
smp_operations.smp_prepare_cpus() callback, which is called at most
once.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/116ef21e5f495432d913315bdc25fbc30edcf2e1.1693409184.git.geert+renesas@glider.be