Add platform error codes

Add error codes for the platform module, to be used by
the setup \ terminate API.
diff --git a/include/mbedtls/platform.h b/include/mbedtls/platform.h
index a40a64f..a1cd689 100644
--- a/include/mbedtls/platform.h
+++ b/include/mbedtls/platform.h
@@ -43,6 +43,9 @@
 #include "platform_time.h"
 #endif
 
+#define MBEDTLS_ERR_PLATFORM_INVALID_DATA    -0x0080 /**< Bad input parameters to the platform function. */
+#define MBEDTLS_ERR_PLATFORM_HW_FAILED       -0x0082 /**< Hardware platform function failed. */
+
 #ifdef __cplusplus
 extern "C" {
 #endif