m53menlo: define ft_board_setup only if CONFIG_IS_ENABLED(OF_LIBFDT)

The function ft_board_setup calls do_fixup_by_path_string
which is only available on CONFIG_IS_ENABLED(OF_LIBFDT).
This prepares for the conversion.

ft_board_setup is only called from image-fdt which is linked by
obj-$(CONFIG_$(SPL_TPL_)OF_LIBFDT) += image-fdt.o

Signed-off-by: Troy Kisky <troykiskyboundary@gmail.com>