nb/intel/x4x: Fix programming CxDRB

Programming CxDRB should be cumulative as explained in "Intel ® 4
Series Chipset Family datasheet".

This does not seem to have any real impact but better do according to
the documentation and what vendor firmware does.

This also removes some dead code.

Change-Id: I7ff3264824c843f84b9eb6c06a06aa3f151fe4b3
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/22911
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
1 file changed