watchdog: omap_wdt: Fix WDT reloading

watchdog: omap_wdt: Fix WDT reloading

The watchdog timer value was never updated in the hardware by this
driver, so the watchdog triggered on some random stale value that
was left in the hardware. The TI SPRUH37C says, quote:

  20.4.3.9 Modifying Timer Count/Load Values and Prescaler Setting
  ...
  After a write access, the load register value and prescaler ratio
  registers are updated immediately, but new values are considered
  only after the next consecutive counter overflow or after a new
  trigger command (the WDT_WTGR register).

This means at least one trigger must happen. The driver probably
depended on someone calling it's .reset() callback, however that
is not guaranteed e.g. if the WDT operates without servicing.

Add this missing trigger.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Cc: Sam Protsenko <semen.protsenko@linaro.org>
Cc: Suniel Mahesh <sunil.m@techveda.org>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>