toshiba_acpi: Add keyboard backlight support

Toshiba laptops equiped with an illuminated keyboard
can operate in two different modes: Auto and FN-Z.

The Auto mode turns on the led on keystrokes and
automatically turns it off after some (configurable)
time the last key was pressed.

The FN-Z mode is used to toggle the keyboard led on/off
by userspace.

This patch adds support to set the desired KBD mode and
timeout via sysfs, creates and registers toshiba::kbd_backlight
led device whenever the mode is set to FN-Z.

The acceptable values for mode are: 1 (Auto) and 2 (Fn-Z)
The time values range are: 1-60 seconds

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