arm64: tegra: Add VCC supply for GPIO expanders on Jetson TX2

The GPIO expanders on Jetson TX2 are powered by the VDD_1V8 and
VDD_3V3_SYS supplies, respectively. Model this in device tree so that
the correct supplies are referenced.

Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>