doc: sl-mx8mm: Remove ATF_LOAD_ADDR export

imx8mm-u-boot.dtsi passes the ATF load address via the
'entry' and 'load' properties.

Remove the step that performs the ATF_LOAD_ADDR export, which is
now unneeded.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Patrick Wildt <patrick@blueri.se>
diff --git a/doc/board/kontron/sl-mx8mm.rst b/doc/board/kontron/sl-mx8mm.rst
index 74ff228..7a4c113 100644
--- a/doc/board/kontron/sl-mx8mm.rst
+++ b/doc/board/kontron/sl-mx8mm.rst
@@ -62,7 +62,6 @@
 .. code-block:: bash
 
    $ make kontron-sl-mx8mm_defconfig
-   $ export ATF_LOAD_ADDR=0x920000
    $ make
 
 Burn the flash.bin to SD card at an offset of 33 KiB: