x86: ivybridge: Provide a dummy SDRAM init for 64-bit

We don't support SDRAM init in 64-bit mode since it is essentially
impossible to get into that mode before SDRAM set up. Provide dummy functions
for now. At some point we will need to pass the SDRAM parameters through from
SPL.

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