dt-bindings: rtc: rtc-sh: Add missing power-domains property

make dtbs_check:

    arch/arm/boot/dts/r7s72100-rskrza1.dt.yaml: rtc@fcff1000: 'power-domains' does not match any of the regexes: 'pinctrl-[0-9]+'

Fix this by documenting the missing power-domains property.
Update the example to match reality.

Fixes: 7ae8b2f5 ("dt-bindings: rtc: rtc-sh: convert bindings to json-schema")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>