ARM: stm32: Add board_early_init_f() to SPL

Add weak implementation of board_early_init_f() hook into the
STM32MP1 SPL. This can be used to read out e.g. configuration
straps before initializing the DRAM.

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>