fdt_support: make fdt_fixup_mtdparts() prototype more specific

The second argument of fdt_fixup_mtdparts() is an opaque pointer,
'void *node_info', hence callers can pass any pointer.

Obviously, fdt_fixup_mtdparts() expects 'struct node_info *'
otherwise, it crashes run-time.

Change the prototype so that it is compile-time checked.

Also, add 'const' qualifier to it so that callers can constify
the struct node_info arrays.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>