imx: imx8mn_evk: Drop DECLARE_GLOBAL_DATA_PTR

There is no users of global data in imx8mn_evk.c, drop it.

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 f20ffbc..1b9dc51 100644
--- a/board/freescale/imx8mn_evk/imx8mn_evk.c
+++ b/board/freescale/imx8mn_evk/imx8mn_evk.c
@@ -3,13 +3,10 @@
  * Copyright 2019 NXP
  */
 
-#include <asm/global_data.h>
 #include <asm/io.h>
 #include <env.h>
 #include <init.h>
 
-DECLARE_GLOBAL_DATA_PTR;
-
 int board_mmc_get_env_dev(int devno)
 {
 	return devno;