powerpc/mpc8548cds: Fix network initialization

Add board_eth_init(). PCIe network card is also supported.
Put RGMII init after tsec_eth_init().
Skip initializing eTSEC3 and eTSEC4 with Carrier boards prior to ver 1.3.

Signed-off-by: Ebony Zhu
Signed-off-by: Zhao Chenhui <chenhui.zhao@freescale.com>