dts: phy: add GPIO number and active state used for phy reset

The GEMGXL_RST line on HiFive Unleashed is pulled low and is
using GPIO number 12. Add these reset-gpio details to dt-node
using which the linux phylib can reset the phy.
Signed-off-by: NSagar Shrikant Kadam <sagar.kadam@sifive.com>
Signed-off-by: NPalmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/boot/dts/sifive/hifive-unleashed-a00.dts

@@ -90,6 +90,7 @@
 	phy0: ethernet-phy@0 {
 		compatible = "ethernet-phy-id0007.0771";
 		reg = <0>;
+		reset-gpios = <&gpio 12 GPIO_ACTIVE_LOW>;
 	};
 };