powerpc/83xx/km: add MV88E6122 switch support for kmvect1

powerpc/83xx/km: add MV88E6122 switch support for kmvect1

kmvect1 has a UEC2 connection to the piggy board and a UEC0 connection
to the switch MV88E6122. This switch has a connection to a frontport
ethernet interface. The ethernet port used for network booting is
automatically selected by u-boot. If a Piggy is plugged, the Piggy
port is selected (UEC2, eth1). If the Piggy isn't present, the
Frontport is selected (UEC0, eth0).

The switch reset is connected to a GPIO on the PRIO3 board FPGA (GPIO28)
and released at startup.

Signed-off-by: Karlheinz Jerg <karlheinz.jerg@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>