Add CPSW DT binding to beagle X15 DTS in order to
get ethernet working with this board.

Note that we're also adding sleep state which will
place all pins in mux mode 15 - which means "driver
off" - thus conserving power.
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>