dt-bindings: interrupt-controller: arm,gic-v3: Describe GICv3 optional properties

Describe the optional GICv3 properties:
- clocks
- clock-names
- power-domains
- resets

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210609155108.16590-1-prabhakar.mahadev-lad.rj@bp.renesas.com