MIPS: DTS: img: Fix schema warnings for pwm-leds

The node names for devices using the pwm-leds driver follow a certain
naming scheme (now).  Parent node name is not enforced, but recommended
by DT project.

Signed-off-by: Alexander Dahl <post@lespocky.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>