powerpc/mpc85xx: Add workaround for DDR erratum A004934

After DDR controller is enabled, it performs a calibration for the
transmit data vs DQS paths. During this calibration, the DDR controller
may make an inaccurate calculation, resulting in a non-optimal tap point.

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>