firmware: arm_scmi: Make references to handle const

Now that all the protocol private variable data have been moved out of
struct scmi_handle, mark all of its references as const.

Link: https://lore.kernel.org/r/20210316124903.35011-32-cristian.marussi@arm.com


Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>