watchdog: add MT7621 watchdog support

This patch adds support for the watchdog core found on newer MediaTek Wifi
SoCs MT7621 and MT7628. There is no symbol for MT7628 as it is a subtype of
MT7620 so we depend on that instead.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

Documentation/devicetree/bindings/watchdog/mt7621-wdt.txt

@@ -0,0 +1,12 @@
+Ralink Watchdog Timers
+
+Required properties:
+- compatible: must be "mediatek,mt7621-wdt"
+- reg: physical base address of the controller and length of the register range
+
+Example:
+
+	watchdog@100 {
+		compatible = "mediatek,mt7621-wdt";
+		reg = <0x100 0x10>;
+	};