Merged in juniskane/nuttx_stm32l4/stm32l1_stm32l4_rtc_update_pr (pull request #514)

STM32L1, STM32L4 RTC: add periodic interrupts, update L1 RTC implementation

* STM32L4 RTC: add support experimental CONFIG_RTC_PERIODIC

* STM32 RTC: separate STM32L1 RTC into a separate file

    STM32L1 RTC is very close to F4 or L4 versions, with two alarms
    and periodic wakeup support so backported L4 peripheral to L1.

* RTC: add periodic alarms to upper and lower halves

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