flash: Rework and add flash device support for STM32L4x SoCs

The STM32L4x SoCs embeds a slightly different embedded flash controller
from the STM32F4x SoCs.

This particular controller has the following properties :
 - Up to 2 512KiB banks divided in 2KiB pages
 - Flash can be accessed in any sizes
 - Flash must be written in 64bit aligned 64bit double-words

The drivers/flash/flash_stm32f4x.c is refactored into a new common
drivers/flash/flash_stm32.c and drivers/flash/flash_stm32l4x.c is
created with the STM32L4x specific functions.

To ease the refactoring and keep common functions, the STM32L4x flash
headers are slightly modified to match the hardware reference naming
and solve compilation issues.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>