mtd: nand: denali: remove unneeded <linux/slab.h> includes

The driver calls devm_kzalloc()/devm_kfree() to allocate/free memory.
They are declared in <linux/device.h>, not in <linux/slab.h>.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>