arm64: dts: armada-3720-espressobin: Document URL for schematic

The schematic of the espressobin is publicly available, add a comment
where to find it.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>

arch/arm64/boot/dts/marvell/armada-3720-espressobin.dts

@@ -6,6 +6,9 @@
  * Romain Perier <romain.perier@free-electrons.com>
  *
  */
+/*
+ * Schematic available at http://espressobin.net/wp-content/uploads/2017/08/ESPRESSObin_V5_Schematics.pdf
+ */
 
 /dts-v1/;