mtd: nand: raw: denali: Assert reset before deassert

Always put the controller in reset, then take it out of reset.
This is to make sure controller always in reset state in both SPL and
proper Uboot.

This is preparation for the next patch to poll for reset completion
(rst_comp) bit after reset.

Tested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Radu Bacrau <radu.bacrau@intel.com>
Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>