leds: syscon: use struct led_init_data when registering

By using struct led_init_data when registering we do not need to parse
`label` DT property. Moreover `label` is deprecated and if it is not
present but `color` and `function` are, LED core will compose a name
from these properties instead.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>