watchdog: ulp_wdog: add driver model for ulp watchdog driver

Enable driver model for ulp watchdog timer. When CONFIG_WDT=y and the
status of device node is "okay", initr_watchdog will be called and
finally calls ulp_wdt_probe() and ulp_wdt_start().

Signed-off-by: Alice Guo <alice.guo@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Stefan Roese <sr@denx.de>