firmware: arm_scmi: Constify ops pointers in scmi_handle

These are never modified, so make them const to allow drivers to make
them const.

Link: https://lore.kernel.org/r/20200906230452.33410-3-rikard.falkeborn@gmail.com


Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>