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 <[e-mail]>
Cc: Simon Guinot <[e-mail]>
Cc: Jason Cooper <[e-mail]>
Signed-off-by: Grant Likely <[e-mail]>