power: pmic: sunxi: use generic AXP SPL driver for AXP305

power: pmic: sunxi: use generic AXP SPL driver for AXP305

The generic AXP SPL driver implementation can cover all regulators we
need for the AXP305.

Add the descriptions for four of the six DC/DC regulators of the AXP305,
and enable that when CONFIG_AXP305_POWER is enabled. We won't need DCDC2
and DCDC3, but by using the position in the array for the index we keep
the code cleaner.
Also remove the old driver, and switch the Makefile to include the new,
generic driver.

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