dt-bindings: interrupt-controller: arm,gic: Document resets property

A generic GIC block embedded in an SoC may be connected to an on-SoC
reset controller.  Hence allow the DTS writer to describe this relation,
by documenting the optional presence of a "reset" property.

This gets rid of "make dtbs_check" warnings like:

    arch/arm/boot/dts/r8a7791-porter.dt.yaml: interrupt-controller@f1001000: 'resets' does not match any of the regexes: '^v2m@[0-9a-f]+$', 'pinctrl-[0-9]+'

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>