arm64: tegra: Properly size register regions for GPU on Tegra194

Memory I/O regions for the GV11B found on Tegra194 are 16 MiB rather
than 256 MiB.

Reported-by: Terje Bergström <tbergstrom@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-By: Terje Bergström <tbergstrom@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>