SPM: Provide an SVC entrance for the unprivileged log

As the UART device is occupied by the privileged log, to reuse the
device, provide an SVC entrance for the unprivileged log.

Change-Id: Ia01f434cdecd2697af7841a1fd9dca672537a834
Signed-off-by: Shawn Shan <Shawn.Shan@arm.com>
3 files changed