coresight: resolve pcsave and etm tracing concurreny issue

Always clear ETMCR[pwrdwn] while enabling trace. This is used as
a logic enable and is a pre-requisite while enabling trace. So
switch to an implementation that does this and removes the save
and restore for pwrdwn and pwrup.

Change-Id: I8a14efe8fd76368ed1e177a701013a504aa1876f
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>