fdt_support: skip MTD node with "disabled" in fdt_fixup_mtdparts()

Currently, fdt_fixup_mtdparts() only checks the compatible property.
It is pointless to fix up the disabled node.

Skip the node if it has the property:

  status = "disabled"

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