remoteproc: scp: add COMPILE_TEST dependency

This will improve this driver's build coverage.

Reported-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>