dt-bindings: net: cdns,macb: added generic PHY and reset mappings for ZynqMP

Updated macb DT binding documentation to reflect the phy-names, phys,
resets, reset-names properties which are now used with ZynqMP GEM
devices, and added a ZynqMP-specific DT example.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Signed-off-by: David S. Miller <davem@davemloft.net>