arm64: dts: marvell: Enable spi0 on the board Armada-3720-db

This commit enables the device node spi0 on the official development
board for the Marvell Armada 3700. It also adds sub-node for the 128Mb
SPI-NOR present on the board.

Signed-off-by: Romain Perier <romain.perier@free-electrons.com>
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>