coresight: use ETMPDCR instead of ETMCR for ETM power/clock control

Use ETMPDCR instead of ETMCR for ETM power/clock enable/disable. ETMPDCR
doesn't have corresponding cp14 access and hence will help with subsequent
multicore ETM support changes for 8974. ETMPDCR is available for PFTv1.1
and ETMv3.5 compliant ETM implementations.

Change-Id: I4b190b963cda29b406971cf075db36213b140495
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>