dt-bindings: pwm: tiecap: Add TI 66AK2G SoC specific compatible

Add a new compatible string "ti,k2g-ecap" to support PWM ECAP IP of
TI 66AK2G SoC.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>