dt-bindings: leds: lp50xx: Document child reg, fix example

The led child reg node is the index within the bank, document that
and update the example accordingly.
The reg property in child node is limited to 0-2 since there are 3 leds
per bank, previous value in example was speculative.

Signed-off-by: Johan Adolfsson <johan.adolfsson@axis.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250617-led-fix-v7-2-cdbe8efc88fa@axis.com


Signed-off-by: Lee Jones <lee@kernel.org>