leds: regulator: Make probeable from device tree

The regulator LED can easily be adapted to probe from the
device tree.

We switch led_classdev_register() to led_classdev_register_ext()
passing some struct led_init_data init_data that we leave NULL
save the fwnode if platform data isn't present so that it will be
populated from the device tree.

If we have platform data we set up the name from the platform
data but using init_data instead.

Cc: Antonio Ospite <ao2@ao2.it>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>