sun6i: Add basic axp221 driver

The A31 uses the AXP221 pmic for various voltages.

Signed-off-by: Oliver Schinagl <oliver@schinagl.nl>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>
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Changes in v2:
-Rebase
Changes in v3:
-Add support for all dldo and aldo-s
-Add Kconfig option to select building AXP221 and to select voltage of
 dldo and aldo-s
Changes in v4:
-Add axp221_setbits helper function
-Use symbolic names for enabled bits in CTRL1 - CTRL3 registers