watchdog: hpe-wdt: Introduce HPE GXP Watchdog

Add support for the HPE GXP Watchdog. The GXP asic contains a full
complement of timers one of which is the watchdog timer. The watchdog
timer is 16 bit and has 10ms resolution. The watchdog is created as a
child device of timer since the same register range is used.

Signed-off-by: Nick Hawkins <nick.hawkins@hpe.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Guenter Roeck <linux@roeck-us.net>