ARM: ux500: move GPIO key configuration to device tree

This moves over the configuration of the GPIO keys (used for
proximity sensor and Hall effect sensor) from the static pin
configuration file to the device tree. As part of the exercise,
implement the GPIO keys properly in a per-UIB file as this
setup actually differs with each UIB.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>