[coreboot] [PATCH] Define unused DIMM1 to 0xFF on Geode GX2 boards

[Nils]

This patch defines the unused DIMM1 to 0xFF to make it obvious it is a bogus 
value.

Signed-off-by: Nils Jacobs

This was requested by Myles.
This is Abuild and boot tested.

Thanks, Nils.