coresight: implement stm hardware event and port enable sysfs nodes

Have STM hardware event and port enable sysfs nodes to allow users to
selectively enable and disable either hardware events or stimulus
ports while STM as a whole is enabled.

Change-Id: I32c23f62a3782487e67eb5e6a9da5a5bf7e11df8
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>