arm64: dts: marvell: add second 1G port on the Armada 8040 DB

Armada 8040 DB is equipped with 4 (2x 10G SFI + 2x 1G RGMII)
ethernet ports of which only one was hitherto enabled.
Because currently mvpp2 driver is capable of supporting only
1G RGMII/SGMII, enable second port from CP slave HW block.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>