coresight: stm: change to amba device driver

Current STM device registers itself as a platform device.

Change it to an amba device driver to align it with upstream
implementation of coresight devices.

Change-Id: I7ff9300e3606d5ffc9a54098f7f5d4d03212fec0
Signed-off-by: Shashank Mittal <mittals@codeaurora.org>