ARM: dts: am335x-baltos: add LED support

All three devices provide GPIO based LEDs named power,
wlan and app.

Place LEDs definition into a separate dtsi file as not all
devices including am335x-baltos.dtsi have the same LED layout.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>