ARM: dts: NSP: Move USB3 PHY to internal MDIO bus

ARM: dts: NSP: Move USB3 PHY to internal MDIO bus

This patch largely replicates Vivek Unune's patch "ARM: dts:
BCM5301X:Make usb3 phy use mdio phy driver"[1] for the NSP platform,
whereby we need to create an mdio-mux to facilitate switches
configured via external MDIO, in this case on the Meraki MX65.

However in doing so, we are creating an overlap with usb3_phy's
ccb-mii range. To resolve this, usb3_phy should be moved to a child
node of the internal MDIO bus. The result is heavily based upon Vivek's
patch. This has also been cross-referenced with Yendapally Reddy's
earlier work which utilised the subsequently dropped brcm,nsp-usb3-phy
driver: "[PATCH v2 4/4] arm: dts: nsp: Add USB nodes to device tree"
[2]. Finally, this change provides conformance to the bcm-ns-usb3-phy
documentation, utilising the required usb3-dmp-syscon property. Note
that support for the deprecated ccb-mii bindings has been dropped as of
"phy: phy-bcm-ns-usb3: drop support for deprecated DT binding"[3].

[1] https://lore.kernel.org/patchwork/patch/933971/
[2] https://www.spinics.net/lists/arm-kernel/msg555132.html
[3] https://lore.kernel.org/linux-devicetree/20201113113423.9466-1-zajec5@gmail.com/

Signed-off-by: Matthew Hagan <mnhagan88@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>