imx: imx8mn_evk: Cleanup headers

Drop unused headers and sort them

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/board/freescale/imx8mn_evk/imx8mn_evk.c b/board/freescale/imx8mn_evk/imx8mn_evk.c
index 6b6fb0a..f20ffbc 100644
--- a/board/freescale/imx8mn_evk/imx8mn_evk.c
+++ b/board/freescale/imx8mn_evk/imx8mn_evk.c
@@ -3,12 +3,10 @@
  * Copyright 2019 NXP
  */
 
-#include <env.h>
-#include <init.h>
 #include <asm/global_data.h>
-#include <miiphy.h>
-#include <netdev.h>
 #include <asm/io.h>
+#include <env.h>
+#include <init.h>
 
 DECLARE_GLOBAL_DATA_PTR;