arm: mvebu: dts: turris_mox: add button and LED nodes

Add nodes for indicator LED and reset button so that board code can
implement board factory reset mechanism.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>