coresight: add STM driver support in upstream implementation

Add STM driver in upstream implementation of Coresight driver.

This change copies drivers/coresight/coresight-stm.c (commit :
90095b2a) to driver/hwtracing/coresight
directory.

Change-Id: Id023bf85df0345205ca8baa6a97ff340d5808aeb
Signed-off-by: Shashank Mittal <mittals@codeaurora.org>