MPC85xx: TQM8548: workaround for erratum DDR 19 and 20

This patch adds the workaround for erratum DDR20 according to MPC8548
Device Errata document, Rev. 1: "CKE signal may not function correctly
after assertion of HRESET". Furthermore, the bug DDR19 is fixed in
processor version 2.1 and the work-around must be removed.

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>