mfd: arizona: Move useful defines into a dt-binding include

Move parts of linux/mfd/arizona/pdata.h and gpio.h into a new file in
the dt-binding directory for use by device tree bindings. This also
makes gpio.h redundant so remove it in the process.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Mark Brown <broonie@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>