x86: Set the DRAM banks to reflect real location

x86: Set the DRAM banks to reflect real location

At present with fsp a single DRAM bank is added which extends to the
whole size of memory. However there is typically only 2GB of memory
available below the 4GB boundary, and this is what is used by U-Boot while
running in 32-bit mode.

Scan the tables to set the banks correct. The first bank is set to memory
below 4GB, and the rest of memory is put into subsequent banks.

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