ASoC: ac97: add gpio chip

The AC97 specification provides a guide for 16 GPIOs in the codecs. If
the gpiolib is compiled in the kernel, declare a gpio chip.

This was tested with a pxa27x board (mioa701) and a wm9713 codec.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Mark Brown <broonie@kernel.org>