sunxi: power: axp809: Fix DCDC4 programming

sunxi: power: axp809: Fix DCDC4 programming

When trying to set the DCDC4 regulator, the code was accidentally
setting the voltage register for DCDC5 (VCC-DRAM). The higher voltage
doesn't harm the DRAM chips, but upsets the Linux regulator driver: when
it tried to correct that, it tripped over a separate DT bug.
The DCDC5 DT limits are 1.425 and 1.575V, which cannot bet set with the
rail's resolution of 50mV. The kernel driver gave up, and made in turn
the system hang, as the PMIC powers essential devices.

Fix the copy&paste bug by using the correct PMIC voltage register.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>