doc: verdin-imx8mm: 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/toradex/verdin-imx8mm.rst b/doc/board/toradex/verdin-imx8mm.rst
index a11c82d..439128a 100644
--- a/doc/board/toradex/verdin-imx8mm.rst
+++ b/doc/board/toradex/verdin-imx8mm.rst
@@ -44,7 +44,6 @@
 .. code-block:: bash
 
     $ export CROSS_COMPILE=aarch64-linux-gnu-
-    $ export ATF_LOAD_ADDR=0x920000
     $ make verdin-imx8mm_defconfig
     $ make