board: st: stpmic1: add function stpmic1_init

Add a function stmpic_init to early initialize the PMIC STPMIC1
- keep vdd on during the reset cycle (to avoid issue when backup battery
  is absent)
- Check if debug is enabled to program PMIC according to the bit

This patch allows to remove the compilation of spl.c file from stm32mp1
board in dh_stm32mp1.

CONFIG_SPL_BOARD_INIT is removed as the new function is called earlier
in SPL, in the function board_early_init_f.

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