CHROMIUM: i2c: chromeos_ec: add I2C pass-through

CHROMIUM: i2c: chromeos_ec: add I2C pass-through

Add an I2C adapter acting as a pass-through to send firmware update EC
commands from userspace (e.g. flashrom).

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
BUG=chrome-os-partner:8865
TEST=on Daisy, run flashrom -p internal:bus=lpc -w ec.bin

Change-Id: I84891ee0af978f1ebabd449d8c459b956ee26e67
Reviewed-on: https://gerrit-int.chromium.org/18001
Reviewed-by: Olof Johansson <olofj@google.com>
Commit-Ready: Vincent Palatin <vpalatin@google.com>
Tested-by: Vincent Palatin <vpalatin@google.com>