arm64: dts: marvell: clearfog-gt-8k: fix switch cpu port node

Explicitly set the switch cpu (upstream) port phy-mode and managed
properties. This fixes the Marvell 88E6141 switch serdes configuration
with the recently enabled phylink layer.

Fixes: a6120833 ("arm64: dts: add support for SolidRun Clearfog GT 8K")
Reported-by: Denis Odintsov <d.odintsov@traviangames.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>