Now that the PMIC has a DM driver and binds device tree subnodes, the
GPIO device can be bound that way, instead of from inside board code.
Since the driver still uses the single set of register definitions from
axpXXX.h (as selected by AXPxxx_POWER), it does not differentiate among
the supported compatibles.
Signed-off-by: Samuel Holland <samuel@sholland.org>