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 <picmaster@mail.bg>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tim Harvey <tharvey@gateworks.com>
Cc: Eric Nelson <eric.nelson@boundarydevices.com>