perf: Add Arm CMN-600 PMU driver

Initial driver for PMU event counting on the Arm CMN-600 interconnect.
CMN sports an obnoxiously complex distributed PMU system as part of
its debug and trace features, which can do all manner of things like
sampling, cross-triggering and generating CoreSight trace. This driver
covers the PMU functionality, plus the relevant aspects of watchpoints
for simply counting matching flits.

Tested-by: Tsahi Zidenberg <tsahee@amazon.com>
Tested-by: Tuan Phan <tuanphan@os.amperecomputing.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>