Deprecate hardware acceleration errors

Deprecate the module-specific XXX_HW_ACCEL_FAILED and
XXX_FEATURE_UNAVAILABLE errors, as alternative implementations should now
return `MBEDTLS_ERR_PLATFORM_HW_FAILED` and
`MBEDTLS_ERR_PLATFORM_FEATURE_UNSUPPORTED`.
diff --git a/include/mbedtls/gcm.h b/include/mbedtls/gcm.h
index d2098eb..93d15ee 100644
--- a/include/mbedtls/gcm.h
+++ b/include/mbedtls/gcm.h
@@ -41,7 +41,10 @@
 #define MBEDTLS_GCM_DECRYPT     0
 
 #define MBEDTLS_ERR_GCM_AUTH_FAILED                       -0x0012  /**< Authenticated decryption failed. */
+
+/* MBEDTLS_ERR_GCM_HW_ACCEL_FAILED is deprecated and should not be used. */
 #define MBEDTLS_ERR_GCM_HW_ACCEL_FAILED                   -0x0013  /**< GCM hardware accelerator failed. */
+
 #define MBEDTLS_ERR_GCM_BAD_INPUT                         -0x0014  /**< Bad input parameters to function. */
 
 #ifdef __cplusplus
@@ -146,9 +149,9 @@
  * \return          \c 0 if the encryption or decryption was performed
  *                  successfully. Note that in #MBEDTLS_GCM_DECRYPT mode,
  *                  this does not indicate that the data is authentic.
- * \return          #MBEDTLS_ERR_GCM_BAD_INPUT if the lengths are not valid.
- * \return          #MBEDTLS_ERR_GCM_HW_ACCEL_FAILED or a cipher-specific
- *                  error code if the encryption or decryption failed.
+ * \return          #MBEDTLS_ERR_GCM_BAD_INPUT if the lengths are not valid or
+ *                  a cipher-specific error code if the encryption
+ *                  or decryption failed.
  */
 int mbedtls_gcm_crypt_and_tag( mbedtls_gcm_context *ctx,
                        int mode,
@@ -185,9 +188,8 @@
  *
  * \return          \c 0 if successful and authenticated.
  * \return          #MBEDTLS_ERR_GCM_AUTH_FAILED if the tag does not match.
- * \return          #MBEDTLS_ERR_GCM_BAD_INPUT if the lengths are not valid.
- * \return          #MBEDTLS_ERR_GCM_HW_ACCEL_FAILED or a cipher-specific
- *                  error code if the decryption failed.
+ * \return          #MBEDTLS_ERR_GCM_BAD_INPUT if the lengths are not valid or
+ *                  a cipher-specific error code if the decryption failed.
  */
 int mbedtls_gcm_auth_decrypt( mbedtls_gcm_context *ctx,
                       size_t length,