coresight: tpdm: Check TPDM status before accessing register

Clock needs to be on when accessing TPDM register.  Clock is
enabled with TPDM enablement process.  Check for TPDM enablement
status before accessing register, return error if TPDM is not
enabled.

Change-Id: I3010fc8c8dc2a1c58d2b5276ab832b2e3b063f08
Signed-off-by: Tingwei Zhang <tingwei@codeaurora.org>