arm: mvebu: turris_omnia: Support old DDR3 training

arm: mvebu: turris_omnia: Support old DDR3 training

Support old DDR3 training code on Turris Omnia, selectable via EEPROM
field.

Users experiencing DDR3 initialization failures or random crashes of the
operating system due to incorrect DDR3 configuration can select the old
DDR3 training implementation to fix those issues by setting the EEPROM
field "Use old DDR training":
  eeprom update "Use old DDR training" 1

Signed-off-by: Marek Behún <kabel@kernel.org>