serial: sprd: Add loopback function support

Add loopback function support for Spreadtrum serial controller.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Link: https://lore.kernel.org/r/1275cd9968f1ceb5ac049cc23f1e508025cd552f.1566375260.git.baolin.wang@linaro.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>