UCLASS_MTD is a better fit for NAND drivers.

Make NAND_DENALI_DT depend on DM_MTD, which is needed to compile
drivers/mtd/mtd-uclass.c

Also, make ARCH_UNIPHIER select DM_MTD because all the defconfig
of this platform enables NAND_DENALI_DT.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>