boards: arm: add nucleo_l4a6zg board

Add STM32 Nucleo-144 board board with STM32L4A6ZG MCU
This board has one of the highest memory densities
(1 MB flash, 320 KB SRAM) of the available Nucleo L4
boards, so it will probably be used frequently for prototyping

Signed-off-by: Tomislav Milkovic <tomislav.milkovic95@gmail.com>