watchdog: of_xilinx_wdt: Remove passing null pointer

clk is an optional property, if clock not defined,
calling clk_prepare_enable() and devm_add_action_or_reset()
are not useful.
so calling these two apis only when clock is present.

Addresses-Coverity:"FORWARD_NULL"

Signed-off-by: Srinivas Neeli <srinivas.neeli@xilinx.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210329161939.37680-5-srinivas.neeli@xilinx.com


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