sunxi: pmic_bus: Clean up preprocessor conditions

Instead of using the SoC symbols to decide the bus type, use whichever
bus driver is actually enabled. This allows collapsing all of the AXP2xx
and AXP8xx variants into one "else" case. It also has the advantage of
falling back to I2C when the other bus drivers are disabled; this works
because all of the PMICs support I2C in addition to other interfaces.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>