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>

board/freescale/imx/ddr/mx6q_4x_mt41j128.cfg

@@ -110,7 +110,7 @@ DATA 4 0x021b0018 0x00081740
 
 DATA 4 0x021b001c 0x00008000
 DATA 4 0x021b000c 0x555A7975
-DATA 4 0x021b0010 0xFF538E64
+DATA 4 0x021b0010 0xFF538F64
 DATA 4 0x021b0014 0x01FF00DB
 DATA 4 0x021b002c 0x000026D2