Revert "mx6: ddr: Allow changing REFSEL and REFR fields"

Revert "mx6: ddr: Allow changing REFSEL and REFR fields"

This reverts commit edf0093732225c2fd0791c3864e9a3eef1f92f19 for
cm_fx6 iMX.6 Solo module as it causes frequent (around 10 percent of
power cycles) board's hangs.

These hangs happen in SPL when BSS is being initialized in SDRAM -
it appear that variables from BSS contain trash values which lead to board
hangs. Looks like that SDRAM doesn't yet finish initialization in these
cases.

Signed-off-by: Maxim Yu. Osipov <mosipov@ilbers.de>
Reviewed-by: Stefano Babic <sbabic@denx.de>