dt-bindings: net: snps,dwmac: add "power-domains" property

Since the stmmac driver already uses pm_runtime*() functions, describe
"power-domains" property in the binding.

Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>