x86: Add Advantech SOM-DB5800/SOM-6867 support

x86: Add Advantech SOM-DB5800/SOM-6867 support

Add support for Advantech SOM-DB5800 with the SOM-6867 installed.
This is very similar to conga-qeval20-qa3-e3845 in that there is a
reference carrier board (SOM-DB5800) with a Baytrail based SoM (SOM-6867)
installed.

Currently supported:
 - 2x UART (From ITE EC on SOM-6867) routed to COM3/4 connectors on
   SOM-DB5800.
 - 4x USB 2.0 (EHCI)
 - Video
 - SATA
 - Ethernet
 - PCIe
 - Realtek ALC892 HD Audio
   Pad configuration for HDA_RSTB, HDA_SYNC, HDA_CLK, HDA_SDO
   HDA_SDI0 is set in DT to enable HD Audio codec.
   Pin defaults for codec pin complexs are not changed.

Not supported:
 - Winbond Super I/O (Must be disabled with jumpers on SOM-DB8500)
 - USB 3.0 (XHCI)
 - TPM

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