arm: mx6: Clamp MMDC and DDR3 clocks for timing calculations

arm: mx6: Clamp MMDC and DDR3 clocks for timing calculations

This is proposal for clamping the MMDC/DDR3 clocks to the maximum supported
frequencies as per imx6 SOC models, and for dynamically calculating valid
clock value based on mem_speed.

Currently the code uses impossible values for mem_speed (1333, 1600 MT/s) for
calculating the DDR timings, and uses fixed clock (528 or 400 MHz) which
doesn't take into account DDR3 memory limitations.

Signed-off-by: Nikolay Dimitrov <[email protected]>
Cc: Fabio Estevam <[email protected]>
Cc: Stefano Babic <[email protected]>
Cc: Tim Harvey <[email protected]>
Cc: Eric Nelson <[email protected]>