dts/bindings/pwm/it8xxx2: remove redundant property pwm-output-frequency

it8xxx2 pwm driver does not handle "pwm-output-frequency" property,
so setting the property in borad.dts is useless.

About PWM output frequency, it can be set by pwm-cells "period",
"pwm-output-frequency" is really redundant.

Signed-off-by: Ruibin Chang <Ruibin.Chang@ite.com.tw>