Documentation: Move common leds properties description to separate file.

There are several drivers that use LEDs and depend on exactly the same
device tree binding. However, the binding documentation has simply been
cut-and-paste into each of the binding documents. Rather than continue
to duplicate it, this patch adds a common led binding document that all
can reference.

Signed-off-by: Marek Belisko <marek.belisko@open-nandra.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>