dt-bindings: watchdog: add mpc8xxx-wdt support

Add description of DT bindings for mpc8xxx-wdt driver which
handles the CPU watchdog timer on the mpc83xx, mpc86xx and mpc8xx.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

Documentation/devicetree/bindings/watchdog/mpc8xxx-wdt.txt

+* Freescale mpc8xxx watchdog driver (For 83xx, 86xx and 8xx)
+
+Required properties:
+- compatible: Shall contain one of the following:
+	"mpc83xx_wdt" for an mpc83xx
+	"fsl,mpc8610-wdt" for an mpc86xx
+	"fsl,mpc823-wdt" for an mpc8xx
+- reg: base physical address and length of the area hosting the
+       watchdog registers.
+		On the 83xx, "Watchdog Timer Registers" area:	<0x200 0x100>
+		On the 86xx, "Watchdog Timer Registers" area:	<0xe4000 0x100>
+		On the 8xx, "General System Interface Unit" area: <0x0 0x10>
+
+Optional properties:
+- reg: additional physical address and length (4) of location of the
+       Reset Status Register (called RSTRSCR on the mpc86xx)
+		On the 83xx, it is located at offset 0x910
+		On the 86xx, it is located at offset 0xe0094
+		On the 8xx, it is located at offset 0x288
+
+Example:
+		WDT: watchdog@0 {
+		    compatible = "fsl,mpc823-wdt";
+		    reg = <0x0 0x10 0x288 0x4>;
+		};