led: bcm6328: read base address in the parent node

In the device tree, the address for the led is located
in the parent node (for exemple leds), not in the led node
(for exemple led@0).

The commit "led: bcm6328: convert to use live dt"
(sha1: 89945517)
change this behaviour and read the address in the led node.

We fix this by reading the base address for led
in the parent node.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>