firmware: arm_scmi: Add debugfs ABI documentation for common entries

Add description of the debugfs SCMI common ABI.

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/20230118121426.492864-12-cristian.marussi@arm.com


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