arm: at91: wdt: Convert watchdog driver to dm/dt

Convert the Watchdog driver for AT91SAM9x processors to support
the driver model and device tree. Changes "CONFIG_AT91SAM9_WATCHDOG"
to new "CONFIG_WDT_AT91" Kconfig option.

Signed-off-by: Prasanthi Chellakumar <prasanthi.chellakumar@microchip.com>