coresight-tpdm: add mux select register support

Add support to configure mux select registers for all subunits.

These registers can be used to select different trace events supported
by a subunit.

Change-Id: I5d5236e4c0cb94e401dfe82eeb91e8fe7e3c566b
Signed-off-by: Shashank Mittal <mittals@codeaurora.org>