mtd: rawnand: Introduce nand_choose_best_sdr_timings()

Extract the logic out of nand_choose_interface_config() to create a
public helper that can be reused by manufacturer drivers. Add the
possibility to provide a specific set of timings.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200529111322.7184-22-miquel.raynal@bootlin.com