net: dsa: microchip: lan937x: disable VPHY support

As described by the microchip article "LAN937X - The required
configuration for the external MAC port to operate at RGMII-to-RGMII
1Gbps link speed." [1]:

"When VPHY is enabled, the auto-negotiation process following IEEE 802.3
standard will be triggered and will result in RGMII-to-RGMII signal
failure on the interface because VPHY will try to poll the PHY status
that is not available in the scenario of RGMII-to-RGMII connection
(normally the link partner is usually an external processor).

Note that when VPHY fails on accessing PHY registers, it will fall back
to 100Mbps speed, it indicates disabling VPHY is optional if you only
need the port to link at 100Mbps speed.

Again, VPHY must and can only be disabled by writing VPHY_DISABLE bit in
the register below as there is no strapping pin for the control."

This patch was tested on LAN9372, so far it seems to not to affect VPHY
based clock crossing optimization for the ports with integrated PHYs.

[1]: https://microchip.my.site.com/s/article/LAN937X-The-required-configuration-for-the-external-MAC-port-to-operate-at-RGMII-to-RGMII-1Gbps-link-speed



Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>