sunxi: pmic_bus: Use the DM PMIC interface when possible

The pmic_bus functions are used in both SPL (for regulator setup) and
U-Boot proper (for regulator setup, SID access, GPIO, and poweroff).

Currently, pmic_bus conflicts with DM_I2C because it uses the legacy I2C
interface. This commit makes pmic_bus dual-compatible with either the
legacy I2C functions or the newly-added PMIC_AXP driver (which uses
DM_I2C). In turn, this allows platforms to start transitioning to DM_I2C
in U-Boot proper, without breaking boards that still depend on the
legacy I2C interface for other reasons.

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>