PM / Domains: Add DT bindings for the R-Mobile System Controller

The Renesas R-Mobile System Controller provides a.o. power management
support, following the generic PM domain bindings in
Documentation/devicetree/bindings/power/power_domain.txt.

For now this supports the R-Mobile A1 (r8a7740) only, but it should be
sufficiently generic to handle other members of the SH-Mobile/R-Mobile
family in the future.

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>