iio: light: apds9960: convert to use maple tree register cache

The maple tree register cache is based on a much more modern data structure
than the rbtree cache and makes optimisation choices which are probably
more appropriate for modern systems than those made by the rbtree cache.

Signed-off-by: Chu Guangqing <chuguangqing@inspur.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>