leds: syscon: Use resource managed variant of device register

We have a MFD driver compiled as module instantiating this driver. When
unloading that module, those LED devices are not removed, which produces
conflicts, when that module is inserted again.

Signed-off-by: Alexander Dahl <ada@thorsis.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>