mtd: nand: add ONFI timing mode to nand_timings converter

Add a converter to retrieve NAND timings from an ONFI NAND timing mode.
At the moment, only SDR NAND timings are supported.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>