soc: litex: Fix compile warning when device tree is not configured

The test robot reported:

   drivers/soc/litex/litex_soc_ctrl.c:143:34: warning: unused variable 'litex_soc_ctrl_of_match' [-Wunused-const-variable]
   static const struct of_device_id litex_soc_ctrl_of_match[] = {
                                    ^
   1 warning generated.

As per the random config device tree is not configured causing the
litex_soc_ctrl_of_match match list to not be used.  This would usually
mean that we cannot even use this driver as it depends on device tree,
but as we also have COMPILE_TEST configured we allow it.

Fix the warning by surrounding the unused variable with an ifdef
CONFIG_OF.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>

drivers/soc/litex/litex_soc_ctrl.c

@@ -140,12 +140,13 @@ struct litex_soc_ctrl_device {
 	void __iomem *base;
 };
 
+#ifdef CONFIG_OF
 static const struct of_device_id litex_soc_ctrl_of_match[] = {
 	{.compatible = "litex,soc-controller"},
 	{},
 };
-
 MODULE_DEVICE_TABLE(of, litex_soc_ctrl_of_match);
+#endif /* CONFIG_OF */
 
 static int litex_soc_ctrl_probe(struct platform_device *pdev)
 {