mtd: nand: denali: allow to override mtd->name from label DT property

Commit 28309572 ("mtd: name the mtd device with an optional
label property") allow us to identify a chip in a user-friendly way.

If nand_set_flash_node() picks up the "label" from DT, let's respect
it.  Otherwise, let it fallback to the current name "denali-nand".

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Suggested-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>