ARM: dts: imx6sll-evk: add debug LED support

On i.MX6SLL EVK board, there is a debug LED controlled
by MX6SLL_PAD_EPDC_VCOM1__GPIO2_IO04 pin, add support
for it.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>