powerpc83xx/km: lock the window size to 2GiB befor fixing sdram size

powerpc83xx/km: lock the window size to 2GiB befor fixing sdram size

If the ram size isn't locked to 2GiB, the get_ram_size() may hang up.

Signed-off-by: Christian Herzig <christian.herzig@keymile.com>
cc: Kim Phillips <kim.phillips@freescale.com>
cc: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Gerlando Falauto <gerlando.falauto@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>