platform/chrome: chromeos_laptop - use I2C notifier to create devices

Instead of using platform device and deferrals to handle the case when i2C
adapters appear late in the game, and not handling device unbinding all
that well, let's switch to using I2C bus notifier to get told when a new
I2C adapter appears in the system, and attempt to add appropriate devices
at that time.

In case when we have 2 Designware adapters in the system (Acer C720),
instead of counting and hoping they get enumerate din the right order,
let's switch to using their PCI devids (slot/function) that should be
stable.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Benson Leung <bleung@chromium.org>