In general, the logic voltage is affected by ddr frequency factors.
We should fix the correct voltage range since assuemd that we have the
ddr frequency driver in mainline in the future.

AFAIK, the 1.8v voltage is used by the SD3.0 card.
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>