mtd: nand: fsmc: update of OF support

Add nand bank selection and timings to the device tree bindings.

Signed-off-by: Mian Yousaf Kaukab <mian.yousaf.kaukab@stericsson.com>
[Added some documentation]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>