watchdog: dw_wdt: pat the watchdog before enabling it

On some dw_wdt implementations the "top" register may be initted to 0
at bootup.  In such a case, each "pat" of the watchdog will reset the
timer to 0xffff.  That's pretty short.

The input clock of the wdt can be any of a wide range of values.  On
an rk3288 system, I've seen the wdt clock be 24.75 MHz.  That means
each tick is ~40ns and we'll count to 0xffff in ~2.6ms.

Because of the above two facts, it's a really good idea to pat the
watchdog after initting the "top" register properly and before
enabling the watchdog.  If you don't then there's no way we'll get the
next heartbeat in time.

Jisheng Zhang fixed this problem on some dw_wdt versions by using the
TOP_INIT feature.  However, the dw_wdt on rk3288 doesn't have TOP_INIT
so it's a good idea to also pat the watchdog manually.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>