ARM: dts: NSP: Fix MX65 MDIO mux warnings

The naming of this node is based upon that of the initial EA9500 dts[1].
However this does not conform with the mdio-mux format, yielding the
following message when running dtbs_check:
mdio-mii-mux: $nodename:0: 'mdio-mii-mux' does not match '^mdio-mux[\\-@]?'

Secondly, this node should be moved to within the axi node and given the
appropriate unit address. This also requires exposing the axi node via a
label in bcm-nsp.dtsi. This fixes the following warning:
Warning (unit_address_vs_reg): /mdio-mii-mux: node has a reg or ranges property, but no unit name

[1]https://patchwork.ozlabs.org/project/linux-imx/patch/20180618174159.86150-1-npcomplete13@gmail.com/#1941353



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