watchdog: wdt-uclass.c: keep track of each device's running state

As a step towards handling all DM watchdogs in watchdog_reset(), use a
per-device flag to keep track of whether the device has been started
instead of a bit in gd->flags.

We will still need that bit to know whether we are past
initr_watchdog() and hence have populated gd->watchdog_dev -
incidentally, that is how it was used prior to commit 9c44ff1c.

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>