The OP-TEE driver finds the correct RPMB device to interact with by iterating over available devices until one is found with a programmed authentication matching the one OP-TEE is using. This enables coexisting users of other RPMBs since the owner can be determined by who knows the authentication key.
Devices in plural? I am unaware of any board with multi eMMC devices soldered. Can you refer me to such a platform?
Thanks, Avri