watchdog: bcm6345: callback start use tick instead of ms

The function bcm6345_wdt_start use the argument timeout
as tick but it should be used as milliseconds.

A clock is added as requirement for this driver.
The frequency of the clock is then used to convert the
millisecond to ticks in the function bcm6345_wdt_start.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Stefan Roese <sr@denx.de>