ramips: Improve Sanlinking D240 config

* The left most mini-PCIe slot (the one attached to SIM2) can be
power-cycled by setting GPIO 0 to high/low.

* The D240 only needs the MT76x2 module, so update makefile to reflect this.
Note that until the default mt7620 target is updated, then kmod-mt76 (and thus
kmod-mt7603) will be selected by default.

v2->v3:
* Indentation error.

v1->v2:
* Rename gpio and remove redundant comment (thanks Piotr Dymacz)

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>

target/linux/ramips/dts/D240.dts

@@ -46,6 +46,17 @@
 		bootargs = "console=ttyS0,115200";
 	};
 
+	gpio-export {
+		compatible = "gpio-export";
+		#size-cells = <0>;
+
+		power_mpcie2 {
+			gpio-export,name = "power_mpcie2";
+			gpio-export,output = <GPIO_ACTIVE_LOW>;
+			gpios = <&gpio0 0 GPIO_ACTIVE_HIGH>;
+		};
+	};
+
 	gpio-leds {
 		compatible = "gpio-leds";