firmware: arm_scmi: Create debugfs files for SCMI communication debug metrics

Now that the basic support to collect the SCMI communication debug
metrics is in place, let us create debugfs files for the same so
that they are accessible to the users/debuggers.

Signed-off-by: Luke Parkin <luke.parkin@arm.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Tested-by: Cristian Marussi <cristian.marussi@arm.com>
Message-Id: <20240805131013.587016-5-sudeep.holla@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>