arm: mvebu: turris_omnia: Read DDR speed from EEPROM

arm: mvebu: turris_omnia: Read DDR speed from EEPROM

Some Turris Omnia boards experience memory issues, and by
experimentation we found that some of these issues can be solved by
slowing DDR speed.

Add a new field in the extended EEPROM information structure, ddr_speed.
Support several values in this field (for now 1066F, 1333H, and the
default, 1600K) and use it to overwrite the DDR topology parameters
used by the DDR training algorithm.

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