Replace the hardcoded addresses for accessing the SYSC PM domain
registers by register offsets, relative to the SYSC base address stored
in struct rmobile_pm_domain.

In the future, the SYSC base address will come from DT.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>