doc: imx8mn_evk: Do not export ATF_LOAD_ADDR

The imx8mn_evk target have been converted to use binman.

With the binman approach the ATF load address is described via
devicetree, so remove the now unneeded instruction of exporting
ATF_LOAD_ADDR.

Signed-off-by: Fabio Estevam <festevam@denx.de>
1 file changed