iio: adc: ad7768-1: convert driver to use regmap

iio: adc: ad7768-1: convert driver to use regmap

Convert the AD7768-1 driver to use the regmap API for register
access. This change simplifies and standardizes register interactions,
reducing code duplication and improving maintainability.

Create two regmap configurations, one for 8-bit register values and
other for 24-bit register values.

Since we are using regmap now, define the remaining registers from 0x32
to 0x34.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Link: https://patch.msgid.link/aec9e5452c1ac16d5379a80dfce97c00d85614a2.1744325346.git.Jonathan.Santos@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>