nb/intel/x4x: Clarify the raminit memory mapping

This replaces magic values by macros and adds some comments to improve
readability.

Adds a convenient function to fetch the test address of a rank.

Also fixes the temporary memory map by changing a write to MCHBAR
0x100 to 0x110, since this is what vendor does. (No difference
observed thus far)

TESTED on DG43GT

Change-Id: I58923e4a8a756f4ae65f759e7d46e03fad39fab7
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/22328
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
3 files changed