ARM: dts: AM33XX: Add PWM backlight DT data to am335x-evmsk

PWM output from ecap2 uses as backlight source. Also adds low threshold
value to have a uniform divisions in brightness-levels scales with
inverse polarity.

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>