ARM: tegra: venice2: Add GK20A GPU DT node

Add the device-tree node for the GK20A GPU and leave it disabled.
It is the responsibility of the bootloader to enable it if the
VPR registers have been programmed such that the GPU can operate.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>