powerpc/82xx: use SDRAM detection for mgcoge2ne

powerpc/82xx: use SDRAM detection for mgcoge2ne

mgcoge2ne was an intermediate step towards mgcoge3ne. One difference is the
smaller SDRAM on mgcoge2ne (128MB). To support both boards with the same
u-boot we use here the SDRAM detection.

This patch enables SDRAM detection between 256MB and 128MB.
So in addition to the existing 256MB geometry:
     4 chips x 8M (13 rows, 10 cols) x 16 bit x 4 banks
we can now also have 128MB geometry:
     4 chips x 4M (13 rows,  9 cols) x 16 bit x 4 banks

Signed-off-by: Gerlando Falauto <gerlando.falauto@keymile.com>