dt-bindings: pwm: tegra: Document Tegra234 PWM

Add compatible for nvidia,tegra234-pwm with nvidia,tegra194-pwm as a
fallback. The PWM controller blocks are identical to the ones found on
the Tegra194 SoC. No driver changes are required and compatible string
"nvidia,tegra194-pwm" will be used as a fallback.

Signed-off-by: Sandipan Patra <spatra@nvidia.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>