arm64: dts: marvell: armada-37xx: declare the COMPHY node

Describe the A3700 COMPHY node. It has three PHYs that can be
configured as follow:
* PCIe or GbE
* USB3 or GbE
* SATA or USB3
Each of them has its own memory area.

Suggested-by: Grzegorz Jaszczyk <jaz@semihalf.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>