iio: adc: ad7380: enable regmap cache

Enable regmap cache, to avoid useless access on spi bus.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Link: https://patch.msgid.link/20250108-ad7380-add-alert-support-v4-2-1751802471ba@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>