mtd: fsmc_nand.c: Use default timings if none are provided in the dts

Without this patch the timings are all set to 0 if not specified in the dts.
With this patch the driver falls back to use the defaults that are already
present in the driver and are known to work okay for some (older) boards.

Tested on a custom SPEAr600 based board.

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Viresh Kumar <viresh.linux@gmail.com>
Cc: Mian Yousaf Kaukab <mian.yousaf.kaukab@stericsson.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>