ARM: socfpga: socrates: add gpio-leds

The SOCrates has three HPS LEDs that can be turned on/off via gpio.
Use the first one has heartbeat and add the other two as free LEDs.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>