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
diff --git a/services/std_svc/rmmd/rmmd_main.c b/services/std_svc/rmmd/rmmd_main.c
index c59e68a..cf5ff7b 100644
--- a/services/std_svc/rmmd/rmmd_main.c
+++ b/services/std_svc/rmmd/rmmd_main.c
@@ -375,6 +375,9 @@
 	case RMMD_ATTEST_GET_PLAT_TOKEN:
 		ret = rmmd_attest_get_platform_token(x1, &x2, x3);
 		SMC_RET2(handle, ret, x2);
+	case RMMD_ATTEST_GET_REALM_KEY:
+		ret = rmmd_attest_get_signing_key(x1, &x2, x3);
+		SMC_RET2(handle, ret, x2);
 	default:
 		WARN("RMMD: Unsupported RMM-EL3 call 0x%08x\n", smc_fid);
 		SMC_RET1(handle, SMC_UNK);