powerpc/82xx: adapt SDRAM settings for mgcoge3ne

The HW guys suggested to change these two values. And these values are
now identical to the values we use on mgcoge.

PSDMR_WRC was set to 1C as it should lead to better performance.

Signed-off-by: Gerlando Falauto <gerlando.falauto@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>