ARM: dts: AM33XX: Add user-leds device tree data to am335x-bone

Add gpio-leds device tree data to am335x-bone device to enable gpio
based user-leds (USR0, USR1, USR2 and USR3) present on BeagleBone.

[koen@dominion.thruhere.net: led0, led1 suggested by koen]
Signed-off-by: AnilKumar Ch <anilkumar@ti.com>
Signed-off-by: Benoit Cousson <b-cousson@ti.com>