firmware: arm_scmi: Introduce get_max_msg_size() helper/accessor

When the agent is sending data to the SCMI platform, the drivers in the
agent could check the maximum message size supported to avoid potential
protocol buffer overflow.

Introduce the helper/accessor function get_max_msg_size() for the same.

Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20240418-pinctrl-scmi-v11-1-499dca9864a7@nxp.com


Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>