clk: tegra: remove bogus PCIE_XCLK

The "pcie_xclk" clock is not actually a clock at all, but rather a reset
domain. Now that the custom Tegra module reset API has been removed, we
can remove the definition of any "clocks" that existed solely to support
it.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Acked-By: Peter De Schrijver <pdeschrijver@nvidia.com>