powerpc: define the fman node for the kmcoge4 DTS

Now that the FMAN mac driver has been merged the fman node is relevant.

The kmcoge4 board implements 3 ethernet interfaces, 1 with a RGMII phy
and 2 with fixed 1 Giga SGMII links.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Scott Wood <oss@buserror.net>