ARM: dts: imx6sl: add fec sleep pinctrl for pin PM state

when system suspend, need to set pins to low power state to
save IO power consumption, there are three states of pinctrl:
"default", "idle" and "sleep". Currently enet supports default
and sleep state.

Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>