coresight: add attr to show the logical cpu number

Add a new attribute to show the paired cpu number of ETM.
This will be useful information for the user and can be
used for eg. to determine which logical cpu to online for
a particular ETM.

Change-Id: I3ace88606b93f93de47e1f1351d6ff5a7cd04cb4
Signed-off-by: Xiaogang Cui <xiaogang@codeaurora.org>