clk: mediatek: mt7981: fix support for pwm3 clock

Add and fix support for pwm3 clock. In the pwm DTSI node we were
actually using PWM2 clock for PWM3. Now that we have correct ID also add
the missing entry of gate and mux to support PWM3 clock.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>