mtd: rawnand: denali_dt: remove single anonymous clock support

Commit 6f1fe97b

 ("mtd: rawnand: denali_dt: add more clocks based
on IP datasheet") introduced a more correct binding that requires
three named clocks.

Now that all upstream DT files migrated over to it, remove the single
anonymous clock support.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Dinh Nguyen <dinguyen@kernel.org>
Acked-by: Dinh Nguyen <dinguyen@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>