net: dsa: mv88e6xxx: support RGMII cmode

net: dsa: mv88e6xxx: support RGMII cmode

Since the probe defaults all interfaces to the highest speed possible
(10GBASE-X in mv88e6393x) before the phy mode configuration from the
devicetree is considered it is currently impossible to use port 0 in
RGMII mode.

This change will allow RGMII modes to be configurable for port 0
enabling port 0 to be configured as RGMII as well as serial depending
on configuration.

Signed-off-by: Marcus Carlberg <marcus.carlberg@axis.com>
Link: https://lore.kernel.org/r/20220822144136.16627-1-marcus.carlberg@axis.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>