firmware: arm_scmi: Add NXP i.MX95 SCMI documentation

Add NXP i.MX95 System Control Management Interface(SCMI) vendor
extensions protocol documentation.

Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Message-Id: <20240823-imx95-bbm-misc-v2-v8-4-e600ed9e9271@nxp.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>