x86: Support machines with >4GB of RAM

x86: Support machines with >4GB of RAM

Some systems have more than 4GB of RAM. U-Boot can only place things below
4GB so any memory above that should not be used. Ignore any such memory so
that the memory size will not exceed the maximum.

This prevents gd->ram_size exceeding 4GB which causes problems for PCI
devices which use DMA.

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