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>

lib/Kconfig

@@ -334,7 +334,7 @@ config SPL_OF_LIBFDT
 config FDT_FIXUP_PARTITIONS
 	bool "overwrite MTD partitions in DTS through defined in 'mtdparts'"
 	depends on OF_LIBFDT
-	default n
+	depends on CMD_MTDPARTS
 	help
 	  Allow overwriting defined partitions in the device tree blob
 	  using partition info defined in the 'mtdparts' environment