drivers/watchdog: ASPEED reference dev tree properties for config

Reference the system device tree when configuring the watchdog
engines. If property 'aspeed,reset_type' is present then set
reset behavior based on the specified value.  This can be one of
three different mutually exclusive values
  * cpu - Reset CPU only on watchdog timeout
  * soc - Reset System on Chip
  * system - Full system reset

No reset can also be specified by indicating:
  * none - No reset, assumes another watchdog is responsible for
           this.

Add optional property 'aspeed,external-signal'. If present then
configure to generate external signal on watchdog timeout.

Signed-off-by: Christopher Bostic <cbostic@linux.vnet.ibm.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>