firmware: arm_scmi: Clarify raw per-channel ABI documentation

Explain more in detail how the per-channel <m> identifier is chosen.

Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20230120122326.1932614-1-cristian.marussi@arm.com


Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>