coresight: register to cpu_pm notifiers for cti save / restore

coresight_cti_ctx_save() and coresight_cti_ctx_restore() are
called directly from lpm-levels driver. These functions will no
longer be called from lpm-levels driver when we move to PSCI
framework.  Add support in coresight-cti driver to rely on cpu_pm
callback for save and restore functionality of CPU CTI configuration.

Change-Id: I715dcc2297709b7d903d9bd7752a68864477fd6c
Signed-off-by: Sarangdhar Joshi <spjoshi@codeaurora.org>