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>