fdt_support: add fdt_copy_fixed_partitions function

Add a new function fdt_copy_fixed_partitions to copy the fixed
partition nodes from U-Boot device tree to Linux kernel
device tree and to dynamically configure the MTD partitions.

This function fdt_copy_fixed_partitions is only based on device tree
with livetree compatible function and replace the function
fdt_fixup_mtdparts based on mtdparts variable.

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