driver: soc: xilinx: fix memory leak in xlnx_add_cb_for_notify_event()

In the Linux kernel, the following vulnerability has been resolved:

driver: soc: xilinx: fix memory leak in xlnx_add_cb_for_notify_event()

The kfree() should be called when memory fails to be allocated for cb_data in xlnx_add_cb_for_notify_event(), otherwise there will be a memory leak, so add kfree() to fix it.