firmware: arm_scmi: Make name_get operations return a const

A few protocol operations are available that returns a pointer to an
internal character array representing resource name. Make those functions
return a const pointer to such array.

Link: https://lore.kernel.org/r/20220330150551.2573938-7-cristian.marussi@arm.com


Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>