Crypto: Rename SFN/Secure Function terms

The Secure Function (SFN) is a new terminology introduced in
FF-M v1.1 to refer to RoT Service callback functions.

The "SFN" in crypto is not the same.

This patch renames the SFN/sfn to srv to avoid confusions.

Change-Id: I2e57332fa8c3e71044f2ff73d70cb94f4863ceb3
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
diff --git a/interface/include/tfm_crypto_defs.h b/interface/include/tfm_crypto_defs.h
index 2eaa28b..c2f4d7c 100644
--- a/interface/include/tfm_crypto_defs.h
+++ b/interface/include/tfm_crypto_defs.h
@@ -34,7 +34,7 @@
  *
  */
 struct tfm_crypto_pack_iovec {
-    uint32_t sfn_id;             /*!< Secure function ID used to dispatch the
+    uint32_t srv_id;             /*!< Crypto service ID used to dispatch the
                                   *   request
                                   */
     uint16_t step;               /*!< Key derivation step */