x86: Add support for the samus chromebook

This adds basic support for chromebook_samus. This is the 2015 Pixel and
is based on an Intel broadwell platform.

Supported so far are:
- Serial
- SPI flash
- SDRAM init (with MRC cache)
- SATA
- Video (on the internal LCD panel)
- Keyboard

Various less-visible drivers are provided to make the above work (e.g. PCH,
power control and LPC).

The platform requires various binary blobs which are documented in the
README. The major missing feature is USB3 since the existing U-Boot support
does not work correctly with Intel XHCI controllers.

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