doc: board: freescale: imx8mp_evk: update to newer versions and change ATF_LOAD_ADDR

doc: board: freescale: imx8mp_evk: update to newer versions and change ATF_LOAD_ADDR

Update imx-atf and firmware-imx to latest released versions.

Update address of ATF_LOAD_ADDR that has changed to 0x970000 in imx-atf
commit 48733cb4e773a7584ced601de9d717efa3d73815.

Add 'O=' to make and build in separate directory as one issue has been noticed
where it was trouble building directly inside u-boot source dir. Restructure the workflow
and copy binaries after defconfig to ensure that build directory is created.

Signed-off-by: Peter Bergin <peter@berginkonsult.se>
Cc: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>
Reviewed-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>