Input: tc3589x-keypad - add missing kerneldoc

This adds two missing kerneldoc entries for the TC3589x
keypad driver.

Signed-off-by: Naga Radhesh <naga.radheshy@stericsson.com>
Reviewed-by: Srinidhi Kasagar <srinidhi.kasagar@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>