dt-bindings: phy: Convert Northstar 2 PCIe PHY to YAML

Convert the Broadcom Northstar 2 PCIe PHY Device Tree binding to YAML
and rename it accordingly in the process since it had nothing to do with
a MDIO mux on the PCI(e) bus. This is a pre-requisite to updating
another binding file to YAML.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20211206180049.2086907-8-f.fainelli@gmail.com


Signed-off-by: Rob Herring <robh@kernel.org>