dt-bindings: arm: Add smc/hvc transport for SCMI

SCMI could use SMC/HVC as tranports. Since there is no standardized
SMC/HVC function identifier for the same, we need to use vendor
specific id. Add the same into the devicetree binding document.

Also add arm,scmi-smc compatible string for smc/hvc transport

Link: https://lore.kernel.org/r/1583673879-20714-2-git-send-email-peng.fan@nxp.com


Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
[sudeep.holla: reworded commit log and subject]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>