dt-bindings: net: Document Qcom QCA807x PHY package

Document Qcom QCA807x PHY package.

Qualcomm QCA807X Ethernet PHY is PHY package of 2 or 5
IEEE 802.3 clause 22 compliant 10BASE-Te, 100BASE-TX and
1000BASE-T PHY-s.

Document the required property to make the PHY package correctly
configure and work.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>