watchdog: at91sam9_wdt: Remove now superfluous wdt start and reset

With the new generic function, the scattered other functions are now
removed to be replaced by the generic one. The new version also enables
the configuration of the watchdog timeout via the DT "timeout-sec"
property (if enabled via CONFIG_OF_CONTROL).

The watchdog servicing is enabled via CONFIG_WATCHDOG.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Eugen Hristev <eugen.hristev@microchip.com>