mx6ul_14x14_evk: Pass refsel and refr fields to avoid hang

mx6ul_14x14_evk: Pass refsel and refr fields to avoid hang

When running a NXP 4.1 kernel with U-Boot mainline on a mx6ul-evk,
we observe a hang when going into the lowest operational point of cpufreq.

This hang issue does not happen on the NXP U-Boot version.

After comparing the SPL DDR initialization against the DCD table
from NXP U-Boot, the key difference that causes the hang is the
MDREF register setting:

DATA 4 0x021B0020 0x00000800

,which means:

REF_SEL = 0 --> Periodic refresh cycle: 64kHz
REFR = 1 ---> Refresh Rate - 2 refreshes

So adjust the MDREF initialization for mx6ul_evk accordingly
to fix the kernel hang issue at low bus frequency.

Reported-by: Eric Nelson <eric@nelint.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Eric Nelson <eric@nelint.com>