zynq-zc702/devicetree: Add LED and GPIO keys back

North/South buttons map to "N" and "S" keys.
LED DS23 defaults to the kernel heartbeat signal.
devicetree: Fix missing address/size pair in flash0

Resolves the following dtc warning:
Warning (reg_format): "reg" property in /amba@0/spi@e000d000/flash@0 has invalid length (4 bytes) (#address-cells == 2, #size-cells == 1)

Applies to both zedboard and zc702.