ARM: dts: AM33XX: Add PWMSS device tree nodes

Add PWMSS device tree nodes in relation with ECAP & EHRPWM DT nodes to
AM33XX SoC family. Also populates device tree nodes for ECAP & EHRPWM by
adding necessary properties like pwm-cells, base reg & set disabled as
status.

Signed-off-by: Philip Avinash <avinashphilip@ti.com>
Reviewed-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Benoit Cousson <benoit.cousson@linaro.org>