Now that this driver can be compiled with COMPILE_TEST, we have no
guarantee that CONFIG_OF will also be defined. When that happens, a
warning about mtk_scp_of_match being defined but unused will be reported
so make sure this variable is only defined if of_match_ptr() actually
uses it.

Fixes: cbd2dca7 remoteproc: scp: add COMPILE_TEST dependency
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Link: https://lore.kernel.org/r/20201102074007.299222-1-acourbot@chromium.orgSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>