mtd: denali: make MTD_NAND_DENALI_DT dependent on OF

The build passes even if CONFIG_OF is undefined, but it makes sense
to let it depend on OF.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>