U-boot is responsible for enabling the GPU DT node after all necessary
configuration (VPR setup for T124) is performed. In order to be able to
check whether this configuration has been performed right before booting
the kernel, make it happen during board_init().

Also move VPR configuration into the more generic gpu.c file, which will
also host other GPU-related functions, and let boards specify
individually whether they need VPR setup or not.

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>