x86: Implement a cache for Memory Reference Code parameters

The memory reference code takes a very long time to 'train' its SDRAM
interface, around half a second. To avoid this delay on every boot we can
store the parameters from the last training sessions to speed up the next.

Add an implementation of this, storing the training data in CMOS RAM and
SPI flash.

Signed-off-by: Simon Glass <sjg@chromium.org>