mtd: nand: Introduce nand_data_interface

Currently we have no data structure to fully describe a NAND timing.
We only have struct nand_sdr_timings for NAND timings in SDR mode,
but nothing for DDR mode and also no container to store both types
of timing.
This patch adds struct nand_data_interface which stores the timing
type and a union of different timings. This can be used to pass to
drivers in order to configure the timing.
Add kerneldoc for struct nand_sdr_timings while touching it anyway.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>