sunxi: axp152: dcdc3 scale is 50mV / step not 25mV / step

Currently uboot wrongly uses 25mV / step for dcdc3, this is a copy and paste
error introduced when adding the axp152_mvolt_to_target during review of the
axp152.c driver. This results in u-boot setting Vddr to 2.3V instead of 1.5V.

This commit fixes this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>