coresight: implement runtime pc save control

Add runtime control for enabling/disabling program counter save feature.
Enabling program counter save feature will enable pc to be saved on
reset but implies ETM being left powered on. So we provide user runtime
control to enable the feature when debugging or disable it while taking
power measurements.

Change-Id: Ib007da851ee7f3b0fac195da62aac7def68cc67a
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>