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

Add a new compatible string "ti,am654-ehrpwm" to support EHRPWM IP on
TI AM654 SoC.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>