doc: board: freescale: advise to use newer atf and firmware

Update branch and version information of ATF and DDR firmware files to
point to latest releases provided by NXP.

This is especially critical for imx8mp evk, as the ATF support for that
SoC is only available in latest releases.

Align all SoCs from imx8m family to use identical revisions of ATF and
DDR firmware.

Signed-off-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>