board: stm32mp1: use fdt_copy_fixed_partitions

board: stm32mp1: use fdt_copy_fixed_partitions

Copy the fixed partition nodes from U-Boot device tree to Linux kernel
device tree to dynamically configure the MTD partitions.

fdt_copy_fixed_partitions is only based on device tree
and replace the function fdt_fixup_mtdparts based on mtdparts variable;
the variable mtdid and mtdparts are not more required.

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