drivers: watchdog: wdt-uclass.c: add a property u-boot, noautostart

Since commit 492ee6b8 ("watchdog: wdt-uclass.c: handle all DM
watchdogs in watchdog_reset()"), all the watchdog are started when
the config WATCHDOG_AUTOSTART.

To avoid a binary choice none/all, a property u-boot,noautostart
may be added in the watchdog node of the u-boot device tree to not
autostart this watchdog.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>