freescale: Include env.h to permit reading the environment

This file reads from the environment but does not include the correct
header. Update it.

Signed-off-by: Simon Glass <sjg@chromium.org>

board/freescale/mx7dsabresd/mx7dsabresd.c

@@ -3,6 +3,7 @@
  * Copyright (C) 2015 Freescale Semiconductor, Inc.
  */
 
+#include <env.h>
 #include <init.h>
 #include <net.h>
 #include <asm/arch/clock.h>