uboot-imx

wandboard: Fix hang when going into low frequency

A kernel hang is observed when running wandboard 3.14 kernel and
going to the lowest operational point of cpufreq:

# ifconfig eth0 down
# echo 1 > /sys/class/graphics/fb0/blank

The problem is caused by incorrect setting of the REFR field
of register MDREF. Setting it to 4 refresh commands per refresh
cycle fixes the hang.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>