arm64: tegra: Drop clock and reset names for XUSB powergates

Drop the clock and reset names for the Tegra210 XUSB powergates because
these are not currently used and not required by the Tegra PMC binding
documentation.

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