net: phy: mv88e61xx: Force CPU port link up

When connecting to from a CPU direct to a 88e6097 typically RGMII is
used. In order for traffic to actually pass we need to force the link up
so the CPU MAC on the other end will see the link.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>