ARM: uniphier: move NAND reset assertion to U-Boot proper from SPL

The comment /* deassert reset */ is wrong. It asserts the reset.

It no longer needs to stay in SPL. The NAND controller reset is
handled  in the driver. So, this assert can be moved to the
board_init() of U-Boot proper.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>