perf/arm-cmn: Refactor DTM handling

Untangle DTMs from XPs into a dedicated abstraction. This helps make
things a little more obvious and robust, but primarily paves the way
for further development where new IPs can grow extra DTMs per XP.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/9cca18b1b98f482df7f1aaf3d3213e7f39500423.1638530442.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>