firmware: arm_scmi: Track number of inflight SCMI transfers

Add a new debug counter, `XFERS_INFLIGHT`, to track the number of
currently active in-flight SCMI message transfers. This helps in
understanding system behavior and diagnosing potential issues with
pending or stuck messages.

The counter is incremented when a transfer is registered as in-flight,
and decremented when it completes and is released.

It is automatically added to debugfs for visibility through SCMI debugfs.

Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Philip Radford <philip.radford@arm.com>
Message-Id: <20250630105544.531723-3-philip.radford@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>