Add pixis_set_sgmii command

The 8544DS and 8572DS platforms support an optional SGMII riser card to
expose ethernet over an SGMII interface.  Once the card is in, it is also
necessary to configure the board such that it uses the card, rather than
the on-board ethernet ports.  This can either be done by flipping dip switches
on the motherboard, or by modifying registers in the pixis.  Either way
requires a reboot.

This adds a command to allow users to choose which ports are routed through
the SGMII card, and which through the onboard ports.  It also allows users
to revert to the current switch settings.

This code does not work on the 8572, as the PIXIS is different.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>