ARM: tegra: enable GPU DT node when appropriate

ARM: tegra: enable GPU DT node when appropriate

T124/210 requires some specific configuration (VPR setup) to
be performed by the bootloader before the GPU can be used.
For this reason, the GPU node in the device tree is disabled
by default. This patch enables the node if U-boot has performed
VPR configuration.

Boards enabled by this patch are T124's Jetson TK1 and Venice2
and T210's P2571.

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