STM32 PVD: Adds support for STM32's Programmable Voltage Detector feature. I...

STM32 PVD: Adds support for STM32's Programmable Voltage Detector feature. I put register access behind CONFIG_STM32_ENERGYLITE as have not checked F1/F2/F4 etc. manuals. Tested on STM32L1. PVD interrupt looks generic, at least #defines it needs are in headers for every chip variant.  By Dmitry Nikolaev, submitted by Juha Niskanen.