ARM: tegra: simplify GPU setup

Enable the GPU node in the system-wide ft_system_setup() hook instead of
the board-specific ft_board_hook(). This allows us to enable GPU per SoC
generation instead of per-board as we did initially.

Reported-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>