doc: imx8mm_evk: update doc after using binman

Update doc after switch to binman to pack images

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/doc/board/freescale/imx8mm_evk.rst b/doc/board/freescale/imx8mm_evk.rst
index a9ccdb7..7fd3d72 100644
--- a/doc/board/freescale/imx8mm_evk.rst
+++ b/doc/board/freescale/imx8mm_evk.rst
@@ -43,13 +43,14 @@
    $ export CROSS_COMPILE=aarch64-poky-linux-
    $ make imx8mm_evk_defconfig
    $ export ATF_LOAD_ADDR=0x920000
-   $ make flash.bin
+   $ make
 
 Burn the flash.bin to MicroSD card offset 33KB:
 
 .. code-block:: bash
 
    $sudo dd if=flash.bin of=/dev/sd[x] bs=1024 seek=33 conv=notrunc
+   $sudo dd if=u-boot.itb of=/dev/sdc bs=1024 seek=384 conv=sync
 
 Boot
 ----