watchdog: wdt-uclass.c: introduce struct wdt_priv

watchdog: wdt-uclass.c: introduce struct wdt_priv

As preparation for having the wdt-uclass provided watchdog_reset()
function handle all DM watchdog devices, and not just the first such,
introduce a uclass-owned struct to hold the reset_period and
next_reset, so these become per-device instead of being static
variables.

No functional change intended.

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