ARM: dts: imx6ul-pico-hobbit: Make the child led nodes standard

Use the same child led node and label name as used in the
imx7d-pico-hobbit board.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>