iio: add mcp4725 I2C DAC driver

v5:
* fix warnings (Jonathan Cameron)

v4:
* remove unused indio_dev pointer in mcp4725_data (Jonathan Cameron)
* use u16 instead of unsigned short in mcp4725_data (Jonathan Cameron)
* #include mcp4725.h from linux/iio/dac/

v3:
* move from staging to drivers/iio
* switch to chan_spec
* dev_get_drvdata() -> dev_to_iio_dev()
* annotate probe() and remove() with __devinit and __devexit

v2 (based on comments from Jonathan Cameron and Lars-Peter Clausen):
* did NOT switch to chan_spec yet
* rebase to staging-next tree, update iio header locations
* dropped dac.h #include, not needed
* strict_strtol() -> kstrtol()
* call iio_device_unregister() in remove()
* everything in one patch

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>