ram: stm32mp1: remove __maybe_unused on stm32mp1_ddr_setup

Since the commit f42045b2 ("stm32mp15: replace CONFIG_TFABOOT when
it is possible") the function stm32mp1_ddr_setup is always called so the
__maybe_unused can be removed.

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