mtd: nand: sunxi: adapt clk_rate to tWB, tADL, tWHR and tRHW timings

Adapt the NAND controller clk rate to the tWB, tADL, tWHR and tRHW
timings instead of returning an error when the maximum clk divisor is
not big enough to provide an appropriate timing.

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