toshiba_acpi: Add ECO mode led support

Newer Toshiba laptops now come with a feature called
ECO Mode, where the system is put in low power consupmtion
state and a green (world shaped with leaves) icon illuminates
indicating that the system is in such power state.

This patch adds support to turn on/off the ECO led by
creating and registering the toshiba::eco_mode led.

Signed-off-by: Azael Avalos <coproscefalo@gmail.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>