KeyMint VTS: police Ed25519 msg size limit

Ed25519 signing operations require the secure world to accumulate the
entirety of the message; consequently, impose a limit on message size
for this operation.

Bug: 194358913
Test: VtsAidlKeyMintTargetTest
Change-Id: Ibfb6a54c1d546b5b4e51f42795d2bb4660add772