ramips: make Netgear R7200 a separate device from R6700v2

With the various variants of Netgear R**** devices, make it more
obvious which image should be used for the R7200.

Signed-off-by: Dale Hui <strokes-races0b@icloud.com>
[provide proper commit message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

target/linux/ramips/dts/mt7621_netgear_r7200.dts

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include "mt7621_netgear_sercomm_bzv.dtsi"
+
+/ {
+	compatible = "netgear,r7200", "mediatek,mt7621-soc";
+	model = "Netgear R7200";
+};
+
+&leds {
+	guest_wifi {
+		gpios = <&gpio_expander 6 GPIO_ACTIVE_LOW>;
+		label = "white:guest_wifi";
+	};
+};