watchdog: of_xilinx_wdt: Add comment to spinlock

Based on checkpatch every spinlock should be documented.
The patch is fixing this issue:
./scripts/checkpatch.pl --strict -f drivers/watchdog/of_xilinx_wdt.c
CHECK: spinlock_t definition without comment
+	spinlock_t spinlock;

Signed-off-by: Srinivas Goud <srinivas.goud@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Srinivas Neeli <srinivas.neeli@xilinx.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210329161939.37680-2-srinivas.neeli@xilinx.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>