tty/vt/keyboard: define LED triggers for VT LED states

Now that input core allows controlling keyboards LEDs via standard LED
subsystem triggers let's switch VT keyboard code to make use of this
feature. We will define the following standard triggers: "kbd-scrollock",
"kbd-numlock", "kbd-capslock", and "kbd-kanalock" which are default
triggers for respective LEDs on keyboards.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Tested-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>