jtagv8: add debug and etm register save-restore support for armv8

Debug and ETM registers do not retain their state on power collapse.
This change provides save-restore support for these registers to allow
debugging and tracing across power collapse. Add this support on ARMv8
targets.

Change-Id: I67fe7c236267ba741b3e926bd3c10658f137611c
Signed-off-by: Aparna Das <adas@codeaurora.org>