sunxi: gpio: Add support for gpio pins on the AXP209 pmic

Some boards use GPIO-s on the pmic, one example of this is the A13-OLinuXino
board, which uses gpio0 of the axp209 for the lcd-power signal.

This commit adds support for gpio pins on the AXP209 pmic, the sunxi_gpio.c
changes are universal, adding gpio support for the other AXP pmics (when
necessary) should be a matter of adding the necessary axp_gpio_foo functions
to their resp. drivers, and add "#define AXP_GPIO" to their header file.

Note this commit only adds support for the non device-model version of the
gpio code, patches for adding support to the device-model version are very
welcome.

The string representation for these gpio-s is AXP0-#, the 0 in the AXP0 prefix
is there in case we need to support gpio-s on more then 1 pmic in the future.
At least A80 boards have 2 pmics, and we may end up needing to support gpio-s
on both.

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