Docs: Define tfm_hal_status_t as enum in HAL

TF-M coding style avoids 'typedef' usage for non-function types.
Define 'tfm_hal_status_t' as enum type in HAL.

Change-Id: Idcd0f35c842030e1a42a1ea57173ec39da3f7f4e
Signed-off-by: Summer Qin <summer.qin@arm.com>
diff --git a/docs/design_documents/hardware_abstraction_layer.rst b/docs/design_documents/hardware_abstraction_layer.rst
index a8b3233..b7e1772 100644
--- a/docs/design_documents/hardware_abstraction_layer.rst
+++ b/docs/design_documents/hardware_abstraction_layer.rst
@@ -129,7 +129,16 @@
 
 .. code-block:: c
 
-  typedef int32_t tfm_hal_status_t
+  enum tfm_hal_status_t {
+      TFM_HAL_ERROR_MEM_FAULT = CHAR_MIN,
+      TFM_HAL_ERROR_MAX_VALUE,
+      TFM_HAL_ERROR_BAD_STATE,
+      TFM_HAL_ERROR_NOT_SUPPORTED,
+      TFM_HAL_ERROR_INVALID_INPUT,
+      TFM_HAL_ERROR_NOT_INIT,
+      TFM_HAL_ERROR_GENERIC,
+      TFM_HAL_SUCCESS = 0
+  };
 
 Error Codes
 ===========
@@ -142,69 +151,37 @@
 ---------------
 Status code to indicate general success.
 
-.. code-block:: c
-
-  #define TFM_HAL_SUCCESS ((tfm_hal_status_t)0)
-
 TFM_HAL_ERROR_GENERIC
 ---------------------
 Status code to indicate an error that does not correspond to any defined failure
 cause.
 
-.. code-block:: c
-
-  #define TFM_HAL_ERROR_GENERIC ((tfm_hal_status_t)-1)
-
 TFM_HAL_ERROR_NOT_INIT
 ----------------------
 Status code to indicate that the module is not initialed.
 
-.. code-block:: c
-
-  #define TFM_HAL_ERROR_NOT_INIT ((tfm_hal_status_t)-2)
-
 TFM_HAL_ERROR_INVALID_INPUT
 ---------------------------
 Status code to indicate that the input is invalid.
 
-.. code-block:: c
-
-  #define TFM_HAL_ERROR_INVALID_INPUT ((tfm_hal_status_t)-3)
-
 TFM_HAL_ERROR_NOT_SUPPORTED
 ---------------------------
 Status code to indicate that the requested operation or a parameter is not
 supported.
 
-.. code-block:: c
-
-  #define TFM_HAL_ERROR_NOT_SUPPORTED ((tfm_hal_status_t)-4)
-
 TFM_HAL_ERROR_BAD_STATE
 -----------------------
 Status code to indicate that the requested action cannot be performed in the
 current state.
 
-.. code-block:: c
-
-  #define TFM_HAL_ERROR_BAD_STATE ((tfm_hal_status_t)-5)
-
 TFM_HAL_ERROR_MAX_VALUE
 -----------------------
 Status code to indicate that the current number has got the max value.
 
-.. code-block:: c
-
-  #define TFM_HAL_ERROR_MAX_VALUE ((tfm_hal_status_t)-6)
-
 TFM_HAL_ERROR_MEM_FAULT
 -----------------------
 Status code to indicate that the memory check failed.
 
-.. code-block:: c
-
-  #define TFM_HAL_ERROR_MEM_FAULT ((tfm_hal_status_t)-7)
-
 --------------
 
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