powerpc/p5040: fix mdio mux for 10G port

Current driver of p5040 assumes 10G port follows 1G port DTSEC5 in
eth port enum structure, it will assign mdio mux depend on this assumption.
This is not true with Fman V3, which added more 1G ports after port DTSEC5
in eth port enum structure, then 10G ports on p5040 will have wrong mdio mux.
So we use dynamic index for 10G ports instead of hardcoded enum value
when doing mdio mux for 10G ports.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>