[POWERPC] 4xx: Add RGMII support for Sequoia 440EPx

This adds RGMII support to Sequoia DTS and sets correct phy-mode
for EMACs. According to Sequoia datasheet, both ethernet ports
are connected to RGMII interface, while ZMII is used only for MDIO.

Signed-off-by: Valentine Barshak <vbarshak@ru.mvista.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>