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/sha512.h b/include/mbedtls/sha512.h
index 0145890..020f95d 100644
--- a/include/mbedtls/sha512.h
+++ b/include/mbedtls/sha512.h
@@ -35,6 +35,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
+/* MBEDTLS_ERR_SHA512_HW_ACCEL_FAILED is deprecated and should not be used. */
 #define MBEDTLS_ERR_SHA512_HW_ACCEL_FAILED                -0x0039  /**< SHA-512 hardware accelerator failed */
 
 #ifdef __cplusplus