soc: renesas: rcar-sysc: Make rcar_sysc_power_is_off() static

As of commit b12ff416 ("ARM: shmobile: r8a7779: Remove legacy PM
Domain remainings"), rcar_sysc_power_is_off() is no longer used from
SoC-specific code.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>