STM23L4 LPTIM PWM support with multi-channel
* arch/arm/src/stm32l4/stm32l4_pwm.c:
fixed some bugs
arch/arm/src/stm32l4/stm32l4_pwm.h:
support LPTIM PWM if PWM multi-channel is selected
Channel mode for LPTIM are not available
* arch/arm/src/stm32l4/Kconfig: add new configuration for STM32L4 LPTIM support
* arch/arm/src/stm32l4/stm32l4_pwm.c: fix warning: resetbit may be used uninitialized
Approved-by: Gregory Nutt <gnutt@nuttx.org>