imx8mm_beacon/imx8mn_beacon: Update build instructions

imx8mm_beacon/imx8mn_beacon: Update build instructions

With binman generating flash.bin, it's not longer necessary to
specify either the location of ATF nor is it necessary to
specify building flash.bin, so let's update the build instructions
to remove those.  While in here, update the revision of ATF and
DDR firmware so both Mini and Nano reference the same revision.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>