nb/intel/x4x: Fix CAS latency detection and max memory detection

Now hardcode maximum memory frequency capability to 800MHz, as
all chipsets in x4x family support PC2-6400 according to the datasheet.
CAS latency detection also relies on this, and has been cleaned up.

Ram initialization does not work with FSB 1333MHz / DDR2 800MHz combination,
so disable this combination for now, and reduce to 667MHz instead.
Still don't know why this is the case, but FSB1333/667 works.

These changes should now allow existing configurations to continue working,
while providing support for previously unworking configurations, due to
previous buggy CAS latency detection code.

TESTED: on GA-G41M-ES2L
CPU: E5200 @ 2.50GHz (FSB 800MHz)

2x 1GB 667MHz hynix	worked @ 667
1x 2GB 800Mhz ARAM	worked @ 800
1x 1GB 667Mhz StarRam	worked @ 667
2x 2GB 800Mhz (generic)	worked @ 800

Change-Id: I1ddd7827ee6fe3d4162ba0546f738a8f9decdf93
Signed-off-by: Damien Zammit <damien@zamaudio.com>
Reviewed-on: https://review.coreboot.org/15818
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

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