mtd: nand: Declare tBERS, tR and tPROG as u64 to avoid integer overflow

All timings in nand_sdr_timings are expressed in picoseconds but some
of them may not fit in an u32.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Fixes: 204e7ecd ("mtd: nand: Add a few more timings to nand_sdr_timings")
Reported-by: Alexander Dahl <ada@thorsis.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: Alexander Dahl <ada@thorsis.com>
Tested-by: Alexander Dahl <ada@thorsis.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>