soc: renesas: rcar-sysc: Provide helpers to power up/down CPUs

Provide helpers to control CPU power areas from platform code, taking
just a CPU index.  This will avoid having to pass full CPU power area
parameter blocks, and thus duplicating information already provided by
SoC-specific SYSC drivers.

This will be used on R-Car H1 only.
Later R-Car generations rely on APMU/RST for CPU power area control.

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