arm: mvebu: set 38x and 39x AVS on lower frequency

Reduce Auto Voltage Scaling VDD limit when core frequency is lower than
1600MHz. This reduces core voltage level from 1.25V to 1.15V, which
saves power.

The code is taken from Marvell's U-Boot 2013.01 revision 18.06.

Reviewed-by: Chris Packham <judge.packham@gmail.com>
Tested-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Stefan Roese <sr@denx.de>