Add Device Tree binding document for the NVIDIA Tegra PEX pad controller. Bug 3621816 Change-Id: I6df186f88c5ea48242472a2c30288d9d0942e679 Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com> Reviewed-on: https://git-master.nvidia.com/r/c/linux-nv-oot/+/2704360 GVS: Gerrit_Virtual_Submit