docs: update messaging method ff-a binding

FF-A v1.2 ALP0 spec specifies that bits[10:9] of the partition
properties descriptor are used to indicate whether a partition can
send/receive messages via FFA_MSG_SEND_DIRECT_REQ2 ABI.

This patch updates the documentation with the description of these
bits and a new size for the messaging-method manifest field.

Signed-off-by: Kathleen Capella <kathleen.capella@arm.com>
Change-Id: I01cea9a7c8020efab71eb4e264e045a9d815bead
diff --git a/docs/secure-partition-manager/ffa-manifest-binding.rst b/docs/secure-partition-manager/ffa-manifest-binding.rst
index c758139..4d41e10 100644
--- a/docs/secure-partition-manager/ffa-manifest-binding.rst
+++ b/docs/secure-partition-manager/ffa-manifest-binding.rst
@@ -105,13 +105,15 @@
      The "compatible" must be the string "arm,ffa-manifest-rx_tx-buffer".
 
 - messaging-method [mandatory]
-   - value type: <u8>
+   - value type: <u16>
    - Specifies which messaging methods are supported by the partition, set bit
      means the feature is supported, clear bit - not supported:
 
-      - Bit[0]: partition can receive direct requests if set
-      - Bit[1]: partition can send direct requests if set
+      - Bit[0]: partition can receive direct requests via FFA_MSG_SEND_DIRECT_REQ ABI if set
+      - Bit[1]: partition can send direct requests via FFA_MSG_SEND_DIRECT_REQ ABI if set
       - Bit[2]: partition can send and receive indirect messages
+      - Bit[9]: partition can receive direct requests via FFA_MSG_SEND_DIRECT_REQ2 ABI if set
+      - Bit[10]: partition can send direct requests via FFA_MSG_SEND_DIRECT_REQ2 ABI if set
 
 - managed-exit
    - value type: <empty>