watchdog: xilinx: Simplify probe and remove functions

Use devm_ helper function to simplify probe and error path.
Move ioremap to the beginning of probe function.

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>