ppc4xx: Fix 4xx enet driver to support 460GT EMAC2+3

This patch fixes a problem with the RGMII setup of the 460GT. The 460GT
has 2 RGMII instances and we need to configure the 2nd RGMII instance
for the EMAC2+3 channels.

Signed-off-by: Stefan Roese <sr@denx.de>