mtd: nand: provide several helpers to do common NAND operations

This is part of the process of removing direct calls to ->cmdfunc()
outside of the core in order to introduce a better interface to execute
NAND operations.

Here we provide several helpers and make use of them to remove all
direct calls to ->cmdfunc(). This way, we can easily modify those
helpers to make use of the new ->exec_op() interface when available.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
[miquel.raynal@free-electrons.com: rebased and fixed some conflicts]
Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>