dt-bindings: pwm: tegra: Add compatible string for Tegra186

Tegra186 has 8 different PWM controllers and each controller has only
one output. Earlier SoC generations have 4 PWM outputs per controller.

Add a device tree compatible string for Tegra186 to be able to
differentiate between the two.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>