Fix define for MPC5121 ADS board.

Originally the Makefile defines CONFIG_ADS5121_REV2 when configuring
for board mpc5121ads_rev2. The config in include/configs/mpc5121ads.h
expects a define CONFIG_MPC5121ADS_REV2 for this.
Therefore there are two defines used but only 1 is defined.
This patch renames references to CONFIG_ADS5121_REV2 to CONFIG_MPC5121ADS_REV2
to have one common define.

This patch fixes faulty mem size detection caused by wrong config in
include/configs/mpc5121ads.h

Signed-off-by: Mark Vels <mark.vels@team-embedded.nl>