ARM: dts: am3517-evm: Add User LEDs and Pushbutton

There are some LED's attached to the GPIO expander, and
there are is a bank of switches attached to the GPIO expander.
This patch associates the LED and the switches to it.

Signed-off-by: Derald D. Woods <woods.technical@gmail.com>
Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>