[coreboot] Some notes about NBFreq / MemFreq on amd (pre-ryzen)

[Ivan Ivanov]

Relative ratio between the NorthBridge Frequency :and: Memory
Frequency - has to be 1.25 : 1 or bigger for proper operation. NBFreq
can be lowered to allow more OC (overclocking) margin for the memory,
or higher to boost the memory performance. When XMP mode enabled - the
CPU ratio, BCLK freq and memory parameters will be optimized
automatically.