coresight: krait pass3 support for etm driver

Bottom 8 bits [7:0] of ETMSYNCFR are reserved on Krait pass3. This
means only bits [11:8] are valid since bits [31:12] are specified
as reserved by PFTv1.1 specification.

Use an appropriate value for the synchronization frequency in light
of this change on Krait pass3.

Change-Id: I5f32ad6546fc8a72e0a222cc90e0f23c9779ee3c
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>