Switches to xtest-clean-common optee-examples-clean-common instead of
xtest-clean optee-examples-clean in the optee-os-clean-common. This is
needed in case the platform make file doesn't provide these targets.

Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>