powerpc/mpc8548: Add workaround for erratum NMG_eTSEC129

Erratum NMG_eTSEC129 (eTSEC86 in MPC8548 document) applies to some early
verion silicons. This workaround detects if the eTSEC Rx logic is properly
initialized, and reinitialize the eTSEC Rx logic.

Signed-off-by: Gong Chen <g.chen@freescale.com>
Signed-off-by: Zhao Chenhui <chenhui.zhao@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>