configs: imx8mp_evk: revert to old ram settings

The 'commit 864ac2cf ("board: imx8mp: Add Engicam
i.Core MX8M Plus EDIMM2.2 Starter Kit")' has changed the imx8mp evk ram
settings from 6GB ram to 2GB.

This changeset reverts the above change.

Signed-off-by: Manoj Sai <abbaraju.manojsai@amarulasolutions.com>
Reported-by  : Peter Bergin <peter@berginkonsult.se>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>