bus: arm-ccn: Correct required arguments for XP PMU events

XP can provide events from two sources: watchpoints, observing traffic
on device ports and PMU looking at internal buses.

Unfortunately the sysfs definition of the PMU events was requiring
port number (instead of bus number) and direction (the buses are
unidirectional), as these fields were shared with the watchpoint event.

Although it does not introduce a major problem (port can be used as
bus alias and direction is simply ignored for XP PMU events), it's
better to fix it now, before external tools start depending on this
behaviour.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>