devicetree: Move NS2 LEDs binding into LEDs directory

leds-ns2.txt is a binding for LEDs, not GPIOs. Move the documentation in
with the rest of the LEDs bindings.

Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Simon Guinot <simon.guinot@sequanux.org>
Cc: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>