dt-bindings: usb: generic-ohci: Document power-domains property

It is quite common for a generic OHCI block to be embedded in an SoC in
its own power domain.  Hence allow the DTS writer to describe the
controller's position in the power hierarchy, by documenting the
optional presence of a "power-domains" property.

This gets rid of "make dtbs_check" warnings like:

    arch/arm64/boot/dts/renesas/r8a774a1-hihope-rzg2m.dt.yaml: usb@ee080000: 'power-domains' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Rob Herring <robh@kernel.org>