x86: Add support for Advantech SOM-6896

x86: Add support for Advantech SOM-6896

Advantech SOM-6896 is a Broadwell U based COM Express Compact Module
Type 6. This patch adds support for it as a coreboot payload.

On board SATA and SPI are functional. On board Ethernet isn't functional
but since it's optional and ties up a PCIe x4 that is otherwise brought
out, this isn't a concern at the moment. USB doesn't work since the
xHCI driver appears to be broken.

Signed-off-by: George McCollister <george.mccollister@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>