firmware: arm_scmi: Add per-channel raw injection support

firmware: arm_scmi: Add per-channel raw injection support

On a system configured with multiple transport channels, expose a few
additional debugfs per-channel entries to allow a user to explicitly select
which transport channel to use for the SCMI message injection.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20230118121426.492864-18-cristian.marussi@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>