drivers/watchdog: Add optional ASPEED device tree properties

Describe device tree optional properties:

* aspeed,reset-type = "cpu|soc|system|none"
One of three different, mutually exclusive, values

"cpu" : ARM CPU reset on signal
"soc" : 'System on chip' reset
"system" : Full system reset

The value can also be set to "none" which indicates that no
reset of any kind is to be done via this watchdog. This assumes
another watchdog on the chip is to take care of resets.

* aspeed,external-signal - Generate external signal (WDT1 and WDT2 only)
* aspeed,alt-boot - Boot from alternate block on signal

Signed-off-by: Christopher Bostic <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Reviewed-by: Guenter Roeck <[e-mail]>
Signed-off-by: Guenter Roeck <[e-mail]>
Signed-off-by: Wim Van Sebroeck <[e-mail]>

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

···
  - reg: physical base address of the controller and length of memory mapped
    region
 
+Optional properties:
+
+ - aspeed,reset-type = "cpu|soc|system|none"
+
+   Reset behavior - Whenever a timeout occurs the watchdog can be programmed
+   to generate one of three different, mutually exclusive, types of resets.
+
+   Type "none" can be specified to indicate that no resets are to be done.
+   This is useful in situations where another watchdog engine on chip is
+   to perform the reset.
+
+   If 'aspeed,reset-type=' is not specfied the default is to enable system
+   reset.
+
+   Reset types:
+
+        - cpu: Reset CPU on watchdog timeout
+
+        - soc: Reset 'System on Chip' on watchdog timeout
+
+        - system: Reset system on watchdog timeout
+
+        - none: No reset is performed on timeout. Assumes another watchdog
+                engine is responsible for this.
+
+ - aspeed,external-signal: If property is present then signal is sent to
+			external reset counter (only WDT1 and WDT2). If not
+			specified no external signal is sent.
+ - aspeed,alt-boot:    If property is present then boot from alternate block.
+
 Example:
 
 	wdt1: watchdog@1e785000 {
 		compatible = "aspeed,ast2400-wdt";
 		reg = <0x1e785000 0x1c>;
+		aspeed,reset-type = "system";
+		aspeed,external-signal;
 	};