feat(ff-a): change manifest messaging method

Align documentation with changes of messaging method for partition
manifest:
      - Bit[0]: support for receiving direct message requests
      - Bit[1]: support for sending direct messages
      - Bit[2]: support for indirect messaging
      - Bit[3]: support for managed exit
Change the optee_sp_manifest to align with the new messaging method
description.

Signed-off-by: Maksims Svecovs <maksims.svecovs@arm.com>
Change-Id: I333e82c546c03698c95f0c77293018f8dca5ba9c
diff --git a/plat/arm/board/fvp/fdts/optee_sp_manifest.dts b/plat/arm/board/fvp/fdts/optee_sp_manifest.dts
index 928d0d3..07235b0 100644
--- a/plat/arm/board/fvp/fdts/optee_sp_manifest.dts
+++ b/plat/arm/board/fvp/fdts/optee_sp_manifest.dts
@@ -25,7 +25,7 @@
 	entrypoint-offset = <0x1000>;
 	xlat-granule = <0>; /* 4KiB */
 	boot-order = <0>;
-	messaging-method = <0>; /* Direct messaging only */
+	messaging-method = <3>; /* Direct messaging only */
 	run-time-model = <1>; /* Run to completion */
 
 	/* Boot protocol */