x86: Add common SDRAM-init code

The code to call the memory reference code is common to several Intel CPUs.
Add common code for performing this init. Intel calls this 'Pre-EFI-Init'
(PEI), where EFI stands for Extensible Firmware Interface.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>