board: stm32mp1: update vddcore in SPL

For board using STPMIC1, the vddcore is provided by BUCK1 of STPMIC1
and need to be updated for 800MHz support and only after the clock
tree initialization.

The VDDCORE voltage value is provided by clock driver, saved in global
variable opp_voltage_mv and udpated in SPL board_early_init_f(),
just after clock tree initialization.

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