google/kefka: add new board as variant of cyan baseboard

Add support for google/kefka (Dell Chromebook 11 3180) as
a variant of the cyan Braswell baseboard.

- Add board-specific code as the new kefka variant
- Add new shared SPD file to baseboard

Sourced from Chromium branch firmware-strago-7287.B,
commit ef41a46: Kefka: Modify USB2 settings to match the eye diagram

Change-Id: Ic6c8c5e5b6029bb99039c64b0182214e93552fa2
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/21573
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Martin Roth <martinroth@google.com>
12 files changed