sunxi: power: Unify axp pmic function names

Stop prefixing the axp functions for setting voltages, etc. with the
model number, there ever is only one pmic driver built into u-boot,
this allows simplifying the callers.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>