mtd: nand: fsmc: remove default timings

When timings are no longer provided by the Device Tree, we now use the
SDR timings specified by the NAND flash, and such SDR timings are always
provided. Therefore, it is no longer necessary to keep "default" timings
in the fmsc driver.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>