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>