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>
Link: https://lore.kernel.org/r/20200915012911.489820-1-acourbot@chromium.org


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>