Aspeed Watchdog Timer

Required properties:
 - compatible: must be one of:
	- "aspeed,ast2400-wdt"
	- "aspeed,ast2500-wdt"
	- "aspeed,ast2600-wdt"

 - 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 specified 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,alt-boot:    If property is present then boot from alternate block.
 - 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,ext-pulse-duration: External signal pulse duration in microseconds

Optional properties for AST2500-compatible watchdogs:
 - aspeed,ext-push-pull: If aspeed,external-signal is present, set the pin's
			 drive type to push-pull. The default is open-drain.
 - aspeed,ext-active-high: If aspeed,external-signal is present and and the pin
			   is configured as push-pull, then set the pulse
			   polarity to active-high. The default is active-low.

Optional properties for AST2500- and AST2600-compatible watchdogs:
 - aspeed,reset-mask: A bitmask indicating which peripherals will be reset if
		      the watchdog timer expires.  On AST2500 this should be a
		      single word defined using the AST2500_WDT_RESET_* macros;
		      on AST2600 this should be a two-word array with the first
		      word defined using the AST2600_WDT_RESET1_* macros and the
		      second word defined using the AST2600_WDT_RESET2_* macros.

Examples:

	wdt1: watchdog@1e785000 {
		compatible = "aspeed,ast2400-wdt";
		reg = <0x1e785000 0x1c>;
		aspeed,reset-type = "system";
		aspeed,external-signal;
	};

	#include <dt-bindings/watchdog/aspeed-wdt.h>
	wdt2: watchdog@1e785040 {
		compatible = "aspeed,ast2600-wdt";
		reg = <0x1e785040 0x40>;
		aspeed,reset-mask = <AST2600_WDT_RESET1_DEFAULT
				     (AST2600_WDT_RESET2_DEFAULT & ~AST2600_WDT_RESET2_LPC)>;
	};