watchdog: xilinx: Allocate private structure per device

Only one watchdog could be used by this driver.
Create driver private data structure and move there
all variables for one instance.

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>