arm64: dts: armada-3720-espressobin: wire up spi flash

This is the storage the machine boots from by default. The partitioning
is taken from the U-Boot that is shipped with the board. There is some
more space on the flash that isn't used.

Tested-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Ellie Reeves <ellierevves@gmail.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>