powerpc: Disable mv643xx Ethernet port 0 on Pegasos

After the last changes, the mv643xx_eth driver now detects
a spurious interface on port 0. Since only port 1 is actually
connected to a PHY, remove its description.

Signed-off-by: Gabriel Paubert <paubert@iram.es>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>