ARM: tegra: Fix compatible string for Tegra114+ timer

The TKE (time-keeping engine) found on Tegra114 and later is no longer
backwards compatible with the version found on Tegra20, so update the
compatible string list accordingly.

Note that while the hardware block is strictly backwards-compatible, an
architectural timer exists on those newer SoCs that is more reliable, so
that should always be preferred.

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