ARM: dts: exynos: Add pinctrl config for Midas keys

This pins are externally pulled up, and so we should explicitly
configure them to disable the SoC-internal pull-downs. Previously
we relied on the bootloader doing this in order to allow the buttons
to function properly.

Signed-off-by: Simon Shields <simon@lineageos.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>