ARM: dts: NSP: Fix MDIO mux node names

ARM: dts: NSP: Fix MDIO mux node names

While functional, the mdio-mux-mmioreg binding does not conform to
Documentation/devicetree/bindings/net/mdio-mux-mmioreg.yaml in that an
mdio-mux compatible is also required. Without this the following output
is observed when running dtbs_check:

mdio-mux@32000: compatible: ['mdio-mux-mmioreg'] is too short

This change brings conformance to this requirement and corresponds
likewise to Rafal Milecki's change to the BCM5301x platform[1].

[1] https://lore.kernel.org/linux-arm-kernel/20210822191256.3715003-1-f.fainelli@gmail.com/T/

Signed-off-by: Matthew Hagan <mnhagan88@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>