feat(rme): add dummy realm attestation key to RMMD

Add a dummy realm attestation key to RMMD, and return it on request.
The realm attestation key is requested with an SMC with the following
parameters:
    * Fid (0xC400001B2).
    * Attestation key buffer PA (the realm attestation key is copied
      at this address by the monitor).
    * Attestation key buffer length as input and size of realm
      attesation key as output.
    * Type of elliptic curve.

Signed-off-by: Tamas Ban <tamas.ban@arm.com>
Signed-off-by: Subhasish Ghosh <subhasish.ghosh@arm.com>
Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Change-Id: I12d8d98fd221f4638ef225c9383374ddf6e65eac