ARM: ux500: improve BU21013 touchpad bindings

In preparation to update to bu21013_tp driver properly annotate GPIOs
property (the INT GPIOs are active low, not open drain), and also define
interrupt lines so we do not have to have special conversion in the driver.

Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>