The ChromeOS EC is connected by LPC only on x86 platforms and no others,
so add a dependency describing that.

But also build the driver if the COMPILE_TEST option is enabled
to have build coverage in other architectures.
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
[olof: reworded commit message]
Signed-off-by: Olof Johansson <olof@lixom.net>