Merged in juniskane/nuttx_stm32l4/stm32l4_dfsdm_pr (pull request #486)

STM32L4 DFSDM: add peripheral, DAC, TIM: small changes

* STM32L4 DAC: do not configure output pin if it is not used

* STM32L4 TIM: fix compilation of timers with complementary outputs when not PWM_MULTICHAN

* STM32L4 DFSDM: peripheral for digital filters for sigma-delta ADCs

    Initial version. Timer trigger support is not completed and there is
    some issue with DMA.

Approved-by: Gregory Nutt <gnutt@nuttx.org>