dt: bcm6858: watchdog should use a 50Mhz clock

The watchdog should use a clock at 50 Mhz, so
instead of using the clock osc (200 Mhz), we
define a reference clock at 50Mhz and use it
for both watchdog.

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