firmware: arm_scmi: Constify struct scmi_voltage_proto_ops

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 <christophe.jaillet@wanadoo.fr>
Message-Id: <2091660c072dd2d5599897243a5d69e89d46ed4d.1753816459.git.christophe.jaillet@wanadoo.fr>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>