x86: Add support for panther (Asus Chromebox)

x86: Add support for panther (Asus Chromebox)

Support running U-Boot as a coreboot payload. Tested peripherals include:

- Video (HDMI and DisplayPort)
- SATA disk
- Gigabit Ethernet
- SPI flash

USB3 does not work. This may be a problem with the USB3 PCI driver or
something in the USB3 stack and has not been investigated So far this is
disabled. The SD card slot also does not work.

For video, coreboot will need to run the OPROM to set this up.

With this board, bare support (running without coreboot) is not available
as yet.

Signed-off-by: Simon Glass <sjg@chromium.org>