dt-bindings: pwm: tiecap: Add TI AM654 SoC specific compatible

Add a new compatible string "ti,am654-ecap" to support PWM ECAP IP of
TI AM654 SoC.

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