soc: qcom: jtagv8-etm: fix race condition during probe

There can be a race condition when one core tries to read its etm data
while coming online, while other core is still writing etm data for same
cpu in jtag_mm_etm_probe.

Protect etm data with hotplug lock to avoid this race condition.

CRs-Fixed: 948917

Change-Id: Ie10a0f09520bf9388860ae2095b3af02c048d7ae
Signed-off-by: Shashank Mittal <mittals@codeaurora.org>