platform/x86: topstar-laptop: add platform device

* add a platform device to support further addition of a led subsystem,

* register the existing input device to this platform device.

Signed-off-by: Guillaume Douézan-Grard <gdouezangrard@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>