Merged in juniskane/nuttx_stm32l4/stm32_rtc_small_patches_pr (pull request #511)

Stm32 rtc small patches

* RTC: canceling an alarm marks it as inactive

* STM32L4, STM32F4, STM32F7 RTC: fix reading alarm value that is more than 24h in future

* STM32F0 RTC: fix backup register count in stm32_rtcc.h

    All other STM32: SHIFTR_SUBFS_MASK was correct in STM32F0 only

* STM32L1: use correct EXTI line definitions

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