config: List cipher modes in alphabetical order

Keeping the cipher modes list in alphabetical order makes it easier to
find things. Move OFB and XTS to their appropriate locations in the
list.
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index e90893f..be83e7f 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -502,20 +502,6 @@
 #define MBEDTLS_CIPHER_MODE_CBC
 
 /**
- * \def MBEDTLS_CIPHER_MODE_XTS
- *
- * Enable Xor-encrypt-xor with ciphertext stealing mode (XTS) for AES.
- */
-#define MBEDTLS_CIPHER_MODE_XTS
-
-/**
- * \def MBEDTLS_CIPHER_MODE_OFB
- *
- * Enable Output Feedback mode (OFB) for symmetric ciphers.
- */
-#define MBEDTLS_CIPHER_MODE_OFB
-
-/**
  * \def MBEDTLS_CIPHER_MODE_CFB
  *
  * Enable Cipher Feedback mode (CFB) for symmetric ciphers.
@@ -530,6 +516,20 @@
 #define MBEDTLS_CIPHER_MODE_CTR
 
 /**
+ * \def MBEDTLS_CIPHER_MODE_OFB
+ *
+ * Enable Output Feedback mode (OFB) for symmetric ciphers.
+ */
+#define MBEDTLS_CIPHER_MODE_OFB
+
+/**
+ * \def MBEDTLS_CIPHER_MODE_XTS
+ *
+ * Enable Xor-encrypt-xor with ciphertext stealing mode (XTS) for AES.
+ */
+#define MBEDTLS_CIPHER_MODE_XTS
+
+/**
  * \def MBEDTLS_CIPHER_NULL_CIPHER
  *
  * Enable NULL cipher.