[WATCHDOG] OMAP fixes: enable clock in probe, trigger timer reload

This patch contains two fixes:

1)In omap_wdt_probe() the watchdog is reset and disabled. This
requires register access and the clks needs to be enabled temporarily

2)In omap_wdt_open() the timer register needs to be reloaded
to trigger a new timer value (the default of 60s)

Tested on OMAP34xx platform (Zoom1)

Reviewed-by: Kevin Hilman <khilman@ti.deeprootsystems.com>
Signed-off-by: Ulrik Bech Hald <ubh@ti.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>