riscv: dts: sifive unmatched: Add gpio poweroff

Some of the GPIO pins on the Unmatched are wire up to control the power
of the board, indicate that in the device tree.

Signed-off-by: Ron Economos <w6rz@comcast.net>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

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

@@ -39,6 +39,11 @@
 		clock-frequency = <RTCCLK_FREQ>;
 		clock-output-names = "rtcclk";
 	};
+
+	gpio-poweroff {
+		compatible = "gpio-poweroff";
+		gpios = <&gpio 2 GPIO_ACTIVE_LOW>;
+	};
 };
 
 &uart0 {