powerpc/mpc8548: Add workaround for erratum NMG_DDR120

Erratum NMG_DDR120 (DDR19 in MPC8548 errata document) applies to some
early version silicons. The default settings of the DDR IO receiver
biasing may not work at cold temperature. When a failure occurs,
a DDR input latches an incorrect value. The workaround will set the
receiver to an acceptable bias point.

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