fdt_support: always define fdt_fixup_mtdparts

SPL code wants fdt_fixup_mtdparts defined as a NOP
when the function isn't linked in.

Prepare for ifdef CONFIG_OF_LIBFDT being converted to
 if CONFIG_IS_ENABLED(OF_LIBFDT)

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