iio: light: veml6030: extend regmap to support caching

iio: light: veml6030: extend regmap to support caching

The configuration registers are not volatile and are not affected
by read operations (i.e. not precious), making them suitable to be
cached in order to reduce the number of accesses to the device.

Add support for caching (RBTREE type).

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://patch.msgid.link/20250119-veml6030-scale-v2-2-6bfc4062a371@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>