watchdog: softdog: remove replaceable ping operation

In watchdog_ping(), 'start' is called automatically when 'ping' function call
is not configured.
Softdog driver has same handling in both cases - start and ping, so 'ping' OPS
can be removed.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>