coresight: tpdm: add read capability for msr

Currently we only support write capability for msr node.
Add read capability to the node in order to get the current
saved msr configuration.

Change-Id: I85d790694d5cd795f2899cc66a6bd2a3b7322876
Signed-off-by: Satyajit Desai <sadesai@codeaurora.org>