Add explicit values to keymaster_key_format_t

Change-Id: I4113286396b16ca687fbd8bc38b2f16063fc1024
diff --git a/include/hardware/keymaster_defs.h b/include/hardware/keymaster_defs.h
index 4649055..770c66e 100644
--- a/include/hardware/keymaster_defs.h
+++ b/include/hardware/keymaster_defs.h
@@ -356,10 +356,10 @@
  * In the future this list will expand greatly to accommodate asymmetric key import/export.
  */
 typedef enum {
-    KM_KEY_FORMAT_X509,   /* for public key export, required */
-    KM_KEY_FORMAT_PKCS8,  /* for asymmetric key pair import, required */
-    KM_KEY_FORMAT_PKCS12, /* for asymmetric key pair import, not required */
-    KM_KEY_FORMAT_RAW,    /* for symmetric key import, required */
+    KM_KEY_FORMAT_X509 = 0,   /* for public key export, required */
+    KM_KEY_FORMAT_PKCS8 = 1,  /* for asymmetric key pair import, required */
+    KM_KEY_FORMAT_PKCS12 = 2, /* for asymmetric key pair import, not required */
+    KM_KEY_FORMAT_RAW = 3,    /* for symmetric key import, required */
 } keymaster_key_format_t;
 
 /**