stm32: Fix error in RTCPRE definition for stm32f4 cpus
stm32: add support for using the HSE clock for RTC

Also:
* it is important to set BDRST whenever changing RTC clock sources,
otherwise the change may not work without forcing a powerdown
* use 32 bit writes to the BDCR register - it is 32 bits wide and there
are important bits in the upper 16 bits
* Assume that the HSE clock is divided down to 1MHz - the recommended
value
* Don't dump registers until after power is given to the backup domain