x86: broadwell: Allow SDRAM init from SPL

x86: broadwell: Allow SDRAM init from SPL

At present, for broadwell, SDRAM is always set up in U-Boot proper since
the 64-bit mode (which uses SDRAM init in SPL) is not supported.

Update the code to allow SDRAM init in SPL instead so that U-Boot proper
can be loaded into SDRAM and run from there. This allows U-Boot to be
compressed to reduce space, since it is not necessary to run it directly
from flash. It could later allow us to support 64-bit U-Boot on broadwell.

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