Improvements for STM32 PWM
arch/arm/src/stm32/stm32_pwm: add support for all PWM modes
arch/arm/src/stm32/stm32_pwm: add interface to change PWM mode
arch/arm/src/stm32/stm32_pwm: refactor pwm_mode_configure()
arch/arm/src/stm32/stm32_pwm: STM32_PWM_CHANx corresponds to the timer channel and STM32_PWM_OUTx corresponds to the timer channel output
arch/arm/src/stm32/stm32_pwm: add CHAN5 and CHAN6 to PWM_TIMx_NCHANNELS
arch/arm/src/stm32/stm32_pwm: calculate the PWM_TIMx_NCHANNELS if CONFIG_STM32_PWM_MULTICHAN is selected
Approved-by: Gregory Nutt <gnutt@nuttx.org>
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