firmware: arm_scmi: Use max-rx-timeout-ms from devicetree

Override default maximum RX timeout with the value picked from the
devicetree, when provided.

Suggested-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Tested-by: Peng Fan <peng.fan@nxp.com>  #i.MX95 19x19 EVK
Message-Id: <20240730144707.1647025-4-cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>