ARM: mvebu_v7_defconfig: fix Ethernet on Clearfog

Compared to kernel 5.0, patches merged for 5.1 added support for A38x'
PHY guarded by a config option which was not enabled by default. As a
result, there was no eth1 and eth2 on a Solid Run Clearfog Base.

Ensure that A38x PHY is enabled on mvebu.

[gregory: issue appeared in 5.1 not in 5.2 and added Fixes tag]

Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Gregory CLEMENT <gregory.clement@bootlin.com>
Cc: Russell King <rmk+kernel@armlinux.org.uk>
Cc: David S. Miller <davem@davemloft.net>
Cc: Maxime Chevallier <maxime.chevallier@bootlin.com>
Fixes: a10c1c81 ("net: marvell: neta: add comphy support")
Cc: stable@kernel.org
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>