Allow to build coresight as a module. This enhances
coresight developer efficiency by allowing the development to
take place exclusively on the target, and without needing to
reboot in between changes.
- Kconfig becomes a tristate, to allow =m
- append -core to source file name to allow module to
be called coresight by the Makefile
- modules can have only one init/exit, so we add the core bus
register/unregister function calls to the etm_perf init/exit
functions, since coresight.c does not have etm_pmu defined.