mtd: rawnand: diskonchip: Implement exec_op()

Implement exec_op() so we can later get rid of the legacy
implementation.

It's worth noting that the new implementation assert/deassert the CE
pin on each operation, which might not be necessary. We also dropped
the extra reset done at chip selection time on DOC2001plus. If it's
needed we really should do something smarter, because having a reset
everytime we access the chip is not that great perf-wise.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200501143917.1388957-5-boris.brezillon@collabora.com