dt-bindings: net: brcm,unimac-mdio: reg-names is optional

The UniMAC MDIO controller integrated into GENET does not provide a
reg-names property since it is optional, reflect that in the binding.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20211206180049.2086907-2-f.fainelli@gmail.com


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