Error codes for hardware accelerator failures

Add MBEDTLS_ERR_XXX_HW_ACCEL_FAILED error codes for all cryptography
modules where the software implementation can be replaced by a hardware
implementation.

This does not include the individual message digest modules since they
currently have no way to return error codes.

This does include the higher-level md, cipher and pk modules since
alternative implementations and even algorithms can be plugged in at
runtime.
diff --git a/include/mbedtls/gcm.h b/include/mbedtls/gcm.h
index f101986..c7f01c3 100644
--- a/include/mbedtls/gcm.h
+++ b/include/mbedtls/gcm.h
@@ -32,6 +32,7 @@
 #define MBEDTLS_GCM_DECRYPT     0
 
 #define MBEDTLS_ERR_GCM_AUTH_FAILED                       -0x0012  /**< Authenticated decryption failed. */
+#define MBEDTLS_ERR_GCM_HW_ACCEL_FAILED                   -0x0013  /**< GCM hardware accelerator failed. */
 #define MBEDTLS_ERR_GCM_BAD_INPUT                         -0x0014  /**< Bad input parameters to function. */
 
 #if !defined(MBEDTLS_GCM_ALT)