platform: add chrome platform directory

It makes sense to split out the Chromebook/Chromebox hardware platform
drivers to a separate subdirectory, since some of it will be shared
between ARM and x86.

This moves over the existing chromeos_laptop driver without making
any other changes, and adds appropriate Kconfig entries for the new
directory. It also adds a MAINTAINERS entry for the new subdir.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>