mtd: nand: davinci: extend description of bindings

Extend bindings for davinci_nand driver to be more clear.
This is clarification only, without semantic changes.

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>