ARM: exynos: snow: Get DP_PD_N & DP_RST_N from dt

ARM: exynos: snow: Get DP_PD_N & DP_RST_N from dt

Add a new node in the device tree which contains entries for DP_PD_N and
DP_RST_N. These values will be fixed up by u-boot to reflect the correct
values for the current board.

BUG=chrome-os-partner:11315
TEST=Tested on snow, ensure the gpio was being correctly harvested and
set

Change-Id: I71b7367c3cbff1407a8d84e0f2a4d7c133ae8ded
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/28036
Reviewed-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Olof Johansson <olofj@chromium.org>