nb/intel/x4x: Implement write leveling

DDR3 adapted a fly-by topology which allows for better signal
integrity but at the same time requires additional calibration. This
is done by settings the targeted rank in write leveling mode while
disabling output buffer on the other ranks. After that the DQS signal
gets sampled over DQ until a transition from high to low is found.

Change-Id: I695969868b4534f87dd1f37244fdfac891a417f0
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/22995
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
3 files changed