arm: stm32mp: move dbgmcu_init call when DT is ready

As the dbgmcu_init use the function bsec_dbgswenable which is based
on the DM and DT, its call can't be done before the spl is initialized
(driver model, DT and malloc) in board_init_f::spl_early_init().

This function call is moved later in spl_board_init().

Fixes: bd3f60d2 ("arm: stm32mp: protect DBGMCU_IDC access with BSEC")

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