soc: renesas: rcar-sysc: Drop legacy handling

Now the R-Car platform code no longer supports DTBs lacking a SYSC
device node in DT, all legacy handling can be dropped from the R-Car
SYSC driver:
  - Make rcar_sysc_ch private to the driver,
  - Make rcar_sysc_power_{down,up}() static (they have been replaced by
    rcar_sysc_power_{down,up}_cpu()),
  - Remove the legacy wrapper rcar_sysc_init(), and the check for double
    initialization (only the early_initcall is left).

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>