iio: adc: ad7768-1: Add GPIO controller support

iio: adc: ad7768-1: Add GPIO controller support

The AD7768-1 has the ability to control other local hardware (such as gain
stages),to power down other blocks in the signal chain, or read local
status signals over the SPI interface.

Add direct mode conditional locks in the GPIO callbacks to prevent register
access when the device is in buffered mode.

This change exports the AD7768-1's four GPIOs and makes them accessible
at an upper layer.

Reviewed-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Acked-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Sergiu Cuciurean <sergiu.cuciurean@analog.com>
Co-developed-by: Jonathan Santos <Jonathan.Santos@analog.com>
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Link: https://patch.msgid.link/8abca580f43cb31d7088d07a7414b5f7efe91ead.1749569957.git.Jonathan.Santos@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>