arm64: tegra: Remove clock-names from PWM nodes

The Tegra PWFM controllers use a single clock, so there's no need for a
clock-names property.

Suggested-by: Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>