bus: arm-ccn: Fix PMU handling of MN

The "Miscellaneous Node" fell through cracks of node initialisation,
as its ID is shared with HN-I.

This patch treats MN as a special case (which it is), adding separate
validation check for it and pre-defining the node ID in relevant events
descriptions. That way one can simply run:

	# perf stat -a -e ccn/mn_ecbarrier/ <workload>

Additionally, direction in the MN pseudo-events XP watchpoint
definitions is corrected to be "TX" (1) as they are defined from the
crosspoint point of view (thus barriers are transmitted from XP to MN).

Cc: stable@vger.kernel.org # 3.17+
Signed-off-by: Pawel Moll <pawel.moll@arm.com>