iio: adc: ad4695: add 2nd regmap for 16-bit registers

iio: adc: ad4695: add 2nd regmap for 16-bit registers

The AD4695 and similar chips have some multibyte registers that have
to be read/written in a single operation. So we need to add a 2nd regmap
for these registers.

These registers are removed from the 8-bit regmap allowable ranges and
AD4695_MAX_REG is dropped since it would be ambiguous now.

debugfs register access is also updated to automatically use the correct
regmap depending on the register address.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20240820-ad4695-gain-offset-v1-1-c8f6e3b47551@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>