firmware: arm_scmi: Constify struct scmi_voltage_proto_ops

Constify struct scmi_voltage_proto_ops since the driver never modifies
it. Placing it in read-only memory (.rodata) improves hardening,
particularly because it contains function pointers.

x86_64 (allmodconfig) size impact:

Before:
text data bss dec hex filename
13142 1808 0 14950 3a66 drivers/firmware/arm_scmi/voltage.o

After:
text data bss dec hex filename
13238 1712 0 14950 3a66 drivers/firmware/arm_scmi/voltage.o

Signed-off-by: Christophe JAILLET <[e-mail]>
Message-Id: <2091660c07[e-mail]>
Signed-off-by: Sudeep Holla <[e-mail]>

drivers/firmware/arm_scmi/voltage.c

···
 	return vinfo->num_domains;
 }
 
-static struct scmi_voltage_proto_ops voltage_proto_ops = {
+static const struct scmi_voltage_proto_ops voltage_proto_ops = {
 	.num_domains_get = scmi_voltage_domains_num_get,
 	.info_get = scmi_voltage_info_get,
 	.config_set = scmi_voltage_config_set,