Add psa_key_agreement_iop_get_num_ops() docs

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/tf-psa-crypto/core/psa_crypto_ecp.c b/tf-psa-crypto/core/psa_crypto_ecp.c
index 95baff6..9e98cc1 100644
--- a/tf-psa-crypto/core/psa_crypto_ecp.c
+++ b/tf-psa-crypto/core/psa_crypto_ecp.c
@@ -592,5 +592,15 @@
 }
 #endif /* MBEDTLS_PSA_BUILTIN_ALG_ECDH */
 
+/****************************************************************/
+/* Interruptible ECC Key Agreement */
+/****************************************************************/
+
+uint32_t psa_key_agreement_iop_get_num_ops(
+    psa_key_agreement_iop_t *operation)
+{
+    (void) operation;
+    return 0;
+}
 
 #endif /* MBEDTLS_PSA_CRYPTO_C */
diff --git a/tf-psa-crypto/include/psa/crypto.h b/tf-psa-crypto/include/psa/crypto.h
index 7a48e3e..75ea8fe 100644
--- a/tf-psa-crypto/include/psa/crypto.h
+++ b/tf-psa-crypto/include/psa/crypto.h
@@ -4852,6 +4852,31 @@
  */
 typedef struct psa_key_agreement_iop_s psa_key_agreement_iop_t;
 
+/**
+ * \brief                       Get the number of ops that a key agreement
+ *                              operation has taken so far. If the operation has
+ *                              completed, then this will represent the number of
+ *                              ops required for the entire operation.
+ *                              After initialization or calling \c
+ *                              psa_key_agreement_iop_abort() on the operation,
+ *                              a value of 0 will be returned.
+ *
+ * \warning                     This is a beta API, and thus subject to change
+ *                              at any point. It is not bound by the usual
+ *                              interface stability promises.
+ *
+ *                              This is a helper provided to help you tune the
+ *                              value passed to \c
+ *                              psa_interruptible_set_max_ops().
+ *
+ * \param operation             The \c psa_key_agreement_iop_t to use. This must
+ *                              be initialized first.
+ *
+ * \return                      Number of ops that the operation has taken so
+ *                              far.
+ */
+uint32_t psa_key_agreement_iop_get_num_ops(psa_key_agreement_iop_t *operation);
+
 /**@}*/
 
 #ifdef __cplusplus