watchdog: xilinx: Convert driver to the watchdog framework

- Remove uneeded headers, fops functions
- Use xilinx_wdt prefix in start/stop/keepalive functions
  and in new structures

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>