platform/chrome: cros_ec_lpc: Mark PROBE_PREFER_ASYNCHRONOUS

This takes on the order of 60ms to probe on some systems, so let it
probe asynchronously. It shouldn't have any dependencies that aren't
handled cleanly.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20221101152132.v2.2.Ib1036816e77aba71ebc16b71f7615c55d054689c@changeid