ARM: dts: Cygnus: Update PCIe PHY node unit name(s)

Update the PCIe PHY node unit name and its sub-nodes to help with
upcoming changes converting the Cygnus PCIe PHY DT binding to YAML and
later the iProc PCIe controller binding to YAML.

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