leds: leds-ns2: move LED modes mapping outside of the driver

On the board n090401 (Seagate NAS 4-Bay), the LED mode mapping (GPIO
values to LED mode) is different from the one used on other boards
supported by the leds-ns2 driver.

With this patch the hardcoded mapping is removed from leds-ns2. Now,
it must be defined either in the platform data (if an old-fashion board
setup file is used) or in the DT node. In order to allow the later, this
patch also introduces a modes-map property for the leds-ns2 DT binding.

Signed-off-by: Vincent Donnefort <vdonnefort@gmail.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>