mtd: rawnand: Pass the CS line to be selected in struct nand_operation

In order to deprecate the ->select_chip hook we need to pass the CS
line a NAND operations are targeting. This is done through the
addition of a cs field to the nand_operation struct.

We also need to keep track of the currently selected target to
properly initialize op->cs, hence the ->cur_cs field addition to the
nand_chip struct.

Note that op->cs is not assigned in nand_exec_op() because we might
rework the way we execute NAND operations in the future (adopt a
queuing mechanism instead of the serialization we have right now).

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Tested-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>