dt: bindings: as3645a: Use LED number to refer to LEDs

Use integers (reg property) to tell the number of the LED to the driver
instead of the node name. While both of these approaches are currently
used by the LED bindings, using integers will require less driver changes
for ACPI support. Additionally, it will make possible LED naming using
chip and LED node names, effectively making the label property most useful
for human-readable names only.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>