drivers: pwm: pwm_stm32: Add 6-PWM support

User can use 6-PWM motor driver in dts like this below:
```dts
pwms =
 // ch1,ch2,ch3,ch1n,ch2n,ch3n
 <&pwm 1 PWM_USEC(50) PWM_POLARITY_NORMAL>, // ch1
 <&pwm 2 PWM_USEC(50) PWM_POLARITY_NORMAL>, // ch2
 <&pwm 3 PWM_USEC(50) PWM_POLARITY_NORMAL>, // ch3
 <&pwm 1 PWM_USEC(50) (PWM_POLARITY_NORMAL|STM32_PWM_COMPLEMENTARY)>,
 <&pwm 2 PWM_USEC(50) (PWM_POLARITY_NORMAL|STM32_PWM_COMPLEMENTARY)>,
 <&pwm 3 PWM_USEC(50) (PWM_POLARITY_NORMAL|STM32_PWM_COMPLEMENTARY)>;
```

Signed-off-by: Savent Gate <savent_gate@outlook.com>