watchdog: gpio_wdt: use __udelay() to avoid recursion

The udelay() function in lib/time.c contains a WATCHDOG_RESET()
call. The only reason this doesn't lead to a catastrophic infinite
recursion is due to the rate-limiting in wdt-uclass.c:

		if (time_after_eq(now, priv->next_reset)) {
			priv->next_reset = now + priv->reset_period;
			wdt_reset(dev);
		}

But this would fall apart if ->next_reset was updated after calling the
device's reset method.

This is needlessly fragile, and it's easy enough to avoid that
recursion in the first place by just using __udelay() directly.

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