SPM: Change log level from variable to string in Cmake

- TFM_SPM_LOG_LEVEL_SILENCE is a special string, not a variable in
  Cmake, change it to string type.
- Exclude TFM_SVC_OUTPUT_UNPRIV_STRING in silence log level.

Change-Id: Idf7ab9955e8568e47d3cdc5ab86ec783945ed086
Signed-off-by: Shawn Shan <Shawn.Shan@arm.com>
5 files changed