ARM: stm32: Implement board coding on AV96

The AV96 board does exist in multiple variants. To cater for all of
them, implement board code handling. There are two GPIOs which code
the type of the board, read them out and use the value to pick the
correct device tree from an fitImage.

Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Change-Id: Iddb330b9a66500495885457cbe17edc0eacaaf43