ARM: stm32: Factor out save_boot_params

The STM32MP15xx platform currently comes with two incompatible
implementations of save_boot_params() weak function override.
Factor the save_boot_params() implementation into common cpu.c
code and provide accessors to read out both ROM API table address
and DT address from any place in the code instead.

Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Marek Vasut <marex@denx.de>