imx: mx6q DDR3 init: Fix tMRD

imx: mx6q DDR3 init: Fix tMRD

MMDC1_MDCFG1.tMRD should be set to max(tMRD, tMOD) for DDR3.

For all DDR3 speed bins:
  tMRD(min) = 4 nCK
  tMOD(min) = max(12 nCK, 15 ns)

Hence, MMDC1_MDCFG1.tMRD should be set to max(12 nCK, 15 ns), which is 12 nCK
at 532 MHz, encoded as 0xB in the bit-field MMDC1_MDCFG1[8:5].

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Tested-by: Eric Nelson <eric.nelson@boundarydevices.com>