coresight: add cpuss debug clock gating support

HW provides flexibility of disabling the debug logic through
software configuration to save dynamic power. Add driver support for
this feature.

CRs-fixed: 934998
Change-Id: I8f8b45f3e37b937199cc2c328311b5df55467421
Signed-off-by: Xiaogang Cui <xiaogang@codeaurora.org>