drm/exynos: dp: support hotplug detection via GPIO

Certain bridge chips use a GPIO to indicate the cable status instead
of the I_DP_HPD pin.  This adds an optional device-tree property,
"samsung,hpd-gpio", to the exynos-dp controller which indicates that
the specified GPIO should be used for hotplug detection.
The GPIO is then set up as an edge-triggered interrupt where the
rising edge indicates hotplug-in and the falling edge indicates hotplug-out.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>