board: stm32mp1: no MTD partitions fixup for serial boot

Remove the update of the MTD partitions in kernel device tree
for serial boot (USB / UART), and the kernel will use the MTD
partitions define in the loaded DTB because U-Boot can't known the
expected flash layout in this case.

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