[coreboot] only 8GB RAM on ThinkPad X230 after commit 7539b8c3

[Marek Behun]

Hello,

with the commit 7539b8c3 "nb/intel/sandybridge: Use common mrc cache
functions" I only have accessible 8 GB of ram on ThinkPad X230,
although I have two 8 GB modules. On the previous commit whole 16 GB is
visible.

Marek