Merge changes from topic "st_ddr_updates" into integration

* changes:
  refactor(st-ddr): move basic tests in a dedicated file
  refactor(st-ddr): reorganize generic and specific elements
  feat(stm32mp1): allow configuration of DDR AXI ports number
  refactor(st-ddr): update parameter array initialization
  feat(st-ddr): add read valid training support
  refactor(stm32mp1): remove the support of calibration result
  fix(st-ddr): correct DDR warnings