coresight: Add explicit architecture dependency

coresight: Add explicit architecture dependency

Coresight hardware is only likely to appear on Arm systems and currently
the core code has Arm-specific barrier operations in it so can't be
built anywhere else so add an explicit dependency saying so.  This will
make no practical difference currently due to the way subsystems are
referenced, the subsystem is only pulled in on arm and arm64, so mainly
serves as documentation in case someone wants to increase build
coverage.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20191104181251.26732-13-mathieu.poirier@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>