mfd: rn5t618: Convert to use maple tree register cache

mfd: rn5t618: 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: Bo Liu <liubo03@inspur.com>
Link: https://lore.kernel.org/r/20240206071314.8721-14-liubo03@inspur.com
Signed-off-by: Lee Jones <lee@kernel.org>