watchdog: at91sam9_wdt: Convert to use the watchdog framework

According to Documentation/watchdog/convert_drivers_to_kernel_api.txt,
remove the file_operations struct, miscdevice, and obsolete includes

Since the at91sam watchdog inherent characteristics, add the watchdog
operations: at91wdt_start, at91wdt_stop and at91wdt_ping.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: linux-watchdog@vger.kernel.org
Cc: linux-kernel@vger.kernel.org