board: stm32mp1: change dfu function to static

Change the dfu functions dfu_otp_read and dfu_pmic_read to static,
this patch avoids warning when compiling with W=1.

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