stm32l4: add initial soc support for stm32l4

Add the initial SoC support for the STM32L4XX family. The code was
tested on STM32L476RG, but should work on any STM32L4XX currently
available.

This implementation was inspired by the stm32f1x implementation.

Change-Id: Id6670bce0c423617284e8467a9c461531f948e0f
Signed-off-by: Fabien Parent <fparent@baylibre.com>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>