fdt_support: make FDT_FIXUP_PARTITIONS depend on CMD_MTDPARTS

fdt_support: make FDT_FIXUP_PARTITIONS depend on CMD_MTDPARTS

fdt_fixup_mtdparts() calls mtdparts_init() and device_find(),
which are defined in cmd/mtdparts.c

The combination of FDT_FIXUP_PARTITIONS=y and CMD_MTDPARTS=n
emits the following link error:

  common/fdt_support.c:903: undefined reference to `mtdparts_init'
  common/fdt_support.c:914: undefined reference to `device_find'

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