net: mv88e61xx: Add support for fixed links

On some boards these switches are wired directly into a SERDES
interface on another Ethernet MAC. Add the ability to specify
these kinds of boards using CONFIG_MV88E61XX_FIXED_PORTS which defines
a bit mask of these fixed ports.

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