mtd: nand: davinci: check required ti,davinci-chipselect property

The property "ti,davinci-chipselect" is required. So we have to check
if it is set.

Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Taras Kondratiuk <taras@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>