dt-bindings: net: realtek,rtl82xx: Document known PHY IDs as compatible strings

Extract known PHY IDs from Linux kernel realtek PHY driver
and convert them into supported compatible string list for
this DT binding document.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>