leds: ns2: support OF probing only, forget platdata

Move forward from platform data to device tree only.

Since commit c7896490 ("leds: ns2: Absorb platform data") the
platform data structure is absorbed into the driver, because nothing
else in the source tree uses it. Since nobody complained and all usage
of this driver is via device tree, change the code to work with device
tree only. As Linus Walleij wrote, the device tree should be the
preferred way forward anyway.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: Simon Guinot <simon.guinot@sequanux.org>
Cc: Simon Guinot <sguinot@lacie.com>
Cc: Vincent Donnefort <vdonnefort@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>