include: trace: Add tracepoint support for inflight xfer count

Enhance the existing SCMI transfer tracepoints by including the current
in-flight transfer count in `scmi_xfer_begin` and `scmi_xfer_end`.

Introduce a new helper `scmi_inflight_count()` to retrieve the active
transfer count from the SCMI debug counters when debug is enabled.

This trace data is useful for visualizing transfer activity over time
and identifying congestion or unexpected behavior in SCMI messaging.

Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Philip Radford <philip.radford@arm.com>
Message-Id: <20250630105544.531723-4-philip.radford@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>