stm32mp1: Add Avenger96 board support ...

Add board support for Avenger96 board from Arrow Electronics. This
board is based on STM32MP157A SoC and is one of the 96Boards Consumer
Edition platform.

More information about this board can be found in 96Boards website:
https://www.96boards.org/product/avenger96/

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Change-Id: Ic905f26c38d03883c6e4ea221b4b275a4b534857