arm64: dts: marvell: improve SPI flash description on Armada 7040-DB

This commit slightly improves the description of the SPI flash
connected to the SPI controller of the Armada 7040, by:

 - Using the more generic "jedec,spi-nor" compatible string, which
   lets the driver auto-detect the exact SPI flash type.

 - Removing the silly comment about the Chip Select, since reg = <0>
   is explicit enough.

 - Switching to the new Device Tree binding to describe flash
   partitions.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>