Input: ab8500-ponkey - add device tree support

Allow the ab8500-ponkey driver to be probed during boot when Device Tree is
enabled.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>